###Weapons To be able to fly takes incredibly strong muscles. So we can assume our succubi are very strong. This would fit well with archery. A highly mobile archer that can shoot from the sky would be fearsome. Obviously they won't be deployed alone but en mass, to shower the enemy formations from above. They could flank the army and even attack the enemy from behind.
Obviously they will also need to engage in close quarter combat if they wish to attack from behind or something similar. For flying a lance or spear makes sense but it’s impossible to carry both the lance and a bow in flight. Unless perhaps you use a smaller compound bow but that’s not really a Medieval European technology.
Those could be carried on the hip in a scabbard. This will exclude the ability to fire from the air as the succubus is already carrying her lance. So lances would make sense for shock troops but our mobile archers need something else. A small shield with an axe or a sword is probably best. Maybe a falchion and a belt worn buckler.
Now both will need armor. But to be able to fly it needs to be light weight. I’m thinking a gamberson. Light compared to most armor and equally less protective. But I don’t think metal armor is possible while maintaining flight. Except the obvious required helmet, always wear a helmet.
###Tactics This depends on their numbers. To be experts at hand to hand combat and master a martial arts it seems they’re trained. Training is an investment. They king is also willing to arm them himself. Again this means they’re not fodder. These are elite troops at least. So they won’t be standing there taking a lance charge.
They will be found on the flanks. Using their high mobility to maneuver around the enemy. Fire arrows in their sides and backs. Use a lance dive in the side of a cavalry charge, disrupting it. In a pinch they could be used to pin down the enemy archers. Landing between them and using their superior training and strength.
Their tails will be less used I think unless it’s magically stronger then armor. Anything but peasants wears armor that should stop your tail. Likely a gamberson with mail on top. Yes that includes the archers. Archery is a skill, it takes quite the investment to get a useful archer. They’re well trained professional warriors that use one of the hardest weapons out there if we’re talking a longbow/warbow. Those take a lot more to muscle to draw then it takes to wield a longsword.
###Terrain While an open field would maximize your mobility it also lacks any sort of cover. This could work with a large pitched battle when the army itself can shield our succubi. In smaller engagement where our succubi are the dominant or only force an open field is more of a liability.
You might want to use your flight to get up to roofs, boulders and trees. Attack the enemy from above then dart out of sight. Descend between them from a rock. Prevent them from having a long clear line of sight.
###Abilities As others mentioned, getting an erection doesn’t seem that bad. Sources mentioning berserkers fighting naked with an erection. That must’ve been a terrifying sight. Your opponents will feel a rush of testosterone, likely suppress all non-life threatening ailments. The body after all has the option to reproduce and that must be seized.
At worst this makes them sinners in their religion and they know have a reason to repent by slaying the vile demons. The eyes might pose an advantage. One on one you look at your opponents face and eyes to see their next move. Your eyes might mess with that. But only in close combat. Against a formation it will do little.