The obvious, but boring, answer is to have him walk up to enemy generals and assassinate them. Everyone trusts him in their camp, the general sees no need to defend himself etc. I'll assume that either Walter is not this powerful, or is too moral to do that because...yeah boring story that way lol.
In fact if he can make people trust him when he says "I mean no harm" he can pretty much shoot and kill anyone without issue. Again this seems boring if you want complex 1 v 1 battles, so lets assume he isn't that powerful.
You mentioned making people distrust themselves. This alone is an absurdly powerful ability. Moral is a major affect on combat effectiveness. if you can make someone have enough self doubt they will run from you without issue, or suck in a fight. However, it to can be a bit bland as a go-to fighting technique, particularly for one V one fights.
So lets look at what you want and see what rules we can create that allow interesting and complex battles, not just easy anticlimactic kills. A few rules come to mind.
Trust magic is limited, the more someone distrusts him the harder it is to get them to trust him and vice versa. He is not so powerful to get nearly-mind control level powers where he can get someone to do anything he wants because they trust him when he says it's a good idea.
If someone knows he is working his magic they can better resist. The degree of power he has depends on how long he has to work, rather they know to expect something, and how strongly someone is resisting; plus how strong their own heroic will power is.
This is to give a few options. On epic 1 v 1 battles his powers are not game breaking because people know to expect them and can resist to a good degree. However, when he is working subtle outside of battle his powers are still quite useful because people aren't actively resisting. Power level is suited for the type of activity he is doing. It also prevents him from ever being too gamebreaking.
So lets look at the types of uses.
One V One battles
It sounds like your thinking anime and manga style battles here, I'm going to work with that for the sort of feel I'm trying to create, so hopefully that's a correct presumption.
The obvious first step is to imply someone with strong will power can better resist him. That way he can walk through mooks (you have to have your heroes beat mooks to show how awesome they are) but the strong characters can better resist because they have stronger will power. Besides shonon series always seem to build up the whole heroic willpower trope to sometimes unreasonable degrees, it may be interesting to justify the trope they all tend to use as an actual mechanic of the world.
I'm also going to assume that Water has some ability to fight outside of his trust magic, so that fight scenes can include the more traditional dodging of attacks and throwing of punches, again trying to keep up with shonan series, and because trust magic is otherwise too boring in a one v one fight (either it works and he wins, or doesn't and he looses, not much middle ground).
In terms of his battles I could see a few techniques he could use.
- Mind games. Get under your enemy skin, mess with their head and freak them out. If his power is dependent on how inclined someone is to believe him then knowing the psychology of his enemy, to know what they are likely to believe, or want to believe, allows him to make better suggestions. In addition giving them a verbal smack down to destroy their confidence, which you can use your trust magic to reinforce, can work to great degrees to destroy moral. It can also justify long verbal banter and soliloquies and all the sort of talk that often happens in these style of shonan stories.
The down side to this approach is that it's sort of a 'bad guy' thing. While intentionally messing with ones confidence like this would presumably work well, and could be made interesting to watch, it definitely feels a bit evil. However, I think you could manage a good guy version if you added a bit of a Warrior Therapist feel to these conversations. Yes he gets into your head and messes with it to undermine your effectiveness, but he does it in a way that ultimately is meant to build you back up.
For example imagine the enemy has someone child raised to be a super soldier TykeBomb, who the badguy manipulated to feel that he was the only one she could trust so she would fight for him. Water may dig into that trust she has for the badguy and point out how much the bad guy doesn't deserve it, which may devastate her and ruin her edge in combat, but would also be the first step to helping her realize she doesn't have to fight for him, which ultimately would help her to heal and go to leading a normal life. He got in her head, messed with her, but in doing so he helped her realize her own delusions (that the person who raised her as a weapon loved her) which helped her to ultimately heal.
He could potentially do this a good bit, destroying their confidence in battle, but only so he can help rebuild them to be better people later. It's much harder to write, but it could make an interesting character, and explain both why he is a good guy and why he has so many followers. Perhaps those he fight often join his side later, and people presume it's because he is mind controlling them but really it's just because they appreciate the way he helped them heal their own mental wounds and made them a better person, they actually want to join him.
- Illusions and deception. The whole look behind you trick may actually work if you can say it in a way that sounds so believable someone actually expects a trap! Perhaps he can do similar, messing with people's beliefs so much that they aren't sure who they are fighting against. In effect he says something so believable that their minds make it real.
Imagine he tells someone he is too fast for them to hit, only he says it so believable they actual accept it. For a few seconds he never seems to be where he should, he almost seems to teleport elsewhere with his speed. Only he isn't any faster, they just believe it so much that for a split second it is true for them.
This gets close to the game breaking territory I mentioned before, but there is an easy way to keep it from being to strong, people can shake off their belief once they have time to think about it for a second, but for a split second they do believe him, and for that split second he has an advantage. It won't last, but it's an opening.
Imagine he is in a fight with a badguy who uses a gun as his preferred weapon. After a volley of fighting, him dodging to try not to be hit while badguy shoots enough bullets to kick up dust the camera freezes on the dustcloud, we watch the dust to clear and find the badguy has his gun pointed at water from only 5 paces away, too close to dodge, and gloats that he has won. Water smiles and says that he would be in trouble, except that the he was counting and the gun should be out of bullets. The badguy gets a scared look on his face and for a split second looks down at the amount counter on the back of the gun, to see he still has plenty of bullets; but during that second Water runs in to close the space between them and lands a punch while the badguy is distracted.
To keep it interesting Water will have to come up with new tricks. He needs lies that have at least a little plausibility so the badguy will trust it for a second, and he only gets a brief advantage before they realize it's a trick. Of course the more he gets into the badguys head the longer it will take for them to realize the trick. And some badguys may simply say they aren't falling for it when he tries a trick, to keep things interesting.
- subtler gambits
The above may be his main weapons, but he would have others to use on a case by case basis. The ability to resist his trust magic is based off of your seeing something that could be manipulation, so if he can work in things you don't notice as manipulation ploys they could stick work.
Another scenario. Water tries a trick like above on a badguy and the bad guy simply says he knows about Water's tricks and won't fall for them, before throwing a surprise attack and Water complains that he hurt his foot/leg avoiding the blow, but he should be fine. Later the badguy barely dodges an attack and Water says he would have landed that attack if he didn't have to be careful about putting weight on his injured leg. Later as the badguy is pressing an advantage Walter accusing the badguy of cheap tactics and points out that if he hadn't been wounded in the first surprise attack he wouldn't be loosing now...
Finally, as the badguy is moving in for the kill on the apparently beaten foe Water leaps forward, using his supposedly wounded leg to propel himself, to move in past the badguys guard and land a nasty blow which ends the fight. It's later revealed that Water's leg was never hurt, but by constantly complaining about his wounded leg he convinced the badguy that it was too badly wounded for Walter to keep fighting (even though he was using the unwounded leg through the fight, the bad guy didn't notice it). Thus the badguy got sloppy and wasn't ready to defend against an attack that used waters supposedly wounded leg. Water points out that the badguy may have been ready to resist his more overt tricks, but was too willing to accept Waters complaints about being wounded as true, he wasn't on guard to defend against those.
Small group battles
Much the same as above, but with some added tricks. Making an enemy distrust something another enemy unit says could be useful. Making them second guess a command by their captain rather then following through can result in a vulnerability to exploit. In addition he can encourage his allies. Moral has a huge bonus in a fight, so just telling everyone that they are going to win this fight, and making them believe it, can really boost moral and thus effectiveness. Sort of like the way bardic music works in an RPG, it's a free buff to those he fights with :)
non-battle strategies
One of the things your notice is that many of the tricks above were short term tricks, people saw through them well. I like that angle, overt trust attacks only work as long as Water is actively using his magic, subtle long-tern effects must be more believable and carefully crafted.
The big thing here is, once again, to get into the psyche of his opponents, but in subtler ways. I think playing mind games by making people think he is manipulating them when he isn't would be a very effective ploy here. Briar patch something by telling someone they totally shouldn't travel through that mountain pass were snipers could easily get you. The enemy is so afraid that your manipulating them that they would go out of their way to do the opposite of what you say and fall right into the trap.
Eventually have him always contact people and tell them things just to mess with them. Have him make up obviously blatant lies, and tell people random things, just to force his opponents to have to wonder what trick he has and second guess what ploy he may have going.
One of the limits of his trust power in large scale battles is that it only works on those he actively manipulated, and he likely can't get to the important people. However, he likely knows quite a bit about how trust and the human psyche works. He may know how to sow disinformation and mistrust without using his powers at all, just by knowing what information to send when and where. I could see plenty of mind games that require no powers at all this way.
Another good trick is to mess with information lies. If he can convince spies or enemies he faced of something that isn't true it will mess with enemy intelligence. He could use this to set up ambushes or bait people by trying to confuse reports of what he is doing and where.
I would suggest giving him an ability to bestow to others some limited ability to use his trust magic, for instance perhaps he makes some sort of dust that you can blow on someone face to make them trust you more. He probably can't make many of these items, but let him do something with it. The reason why is because he can't use his abilities too well for large scale operations because everyone will recognize him and attack on sight, before he has a time to manipulate them. So his picking a select few to do his subtler work for him, and gifting them with some bonus that will help them manipulate others, will allow him to extend his reach beyond where he can physically go, and allow for more mistrust in enemy lines that can be exploited. However, I would make sure you set clear limits on what he can do here, make it clear that one or two individuals he can aid, but he can't give everyone on his side insta-trust goggles or anything like that; no mass producing these tricks.
