Let's imagine an alien life form attacking Earth.
Their technology and their number do not overtake very much human ones, but they have a very powerful weapon: when a human looks right at one of them, he/she finds it so cute that he/she loses their anger and will to fight, even if there is a weapon pointed on himself/herself.
Even when it goes out of the field of view, a memory is left of it and the human remembers it like a good, friendly and harmless creature, and won't wish to try to fight it except for very personal reasons. (If a friend of him/her was killed during a fight, he/she will still want to fight back.) This feeling never wears off by itself, but psychological counseling may help to wear it off, like a regular mental trouble.
They use this weapon in close combat, because doing so nobody can shoot at them directly, and as a tool before invasion too, by filling the human world with photos and videos of them as much as possible via long-range broadcasting, mainly through the Internet.
So now let's suppose the invasion plan already started. The first step of the attack plan, which is to broadcast photos and videos of them through the civilization to reduce potential human aggression to their assault, has already started. But there is still some weeks before the full attack, and a scientist group discovered the truth. Unfortunately, almost 100% of the human population already watched images of those aliens and started to feel tenderness for them.
How can they prepare the world for the final attack, weapons or defense systems for a creature that they must never watch?
(I'm looking only for specific methods against their special weapon.)
Keep in mind it is probable they will display pictures of them on huge screens, paint it on any tank or vehicle, and display as many images as possible of them in every battle to try to affect as many human soldiers as possible.
Even a look through a screen or a picture can affect a human, but it is still less efficient than a direct look on an alien itself.
Of course, this power doesn't affect machines.
Almost any other feature of the aliens (technology, military strategy level, care for casualties, etc) are similar to humans, to be simple.
I'm not looking for a complete defense plan, just a special weapon/tactic that could be used against this precise opponent and its special weapon, in order to not be too broad.