Some Facts you should known with Illusions..

- Don't get hit (disguise as a Fighting Type or Poison Type to bait Psychic Type Moves to be useless) otherwise the Disguise is blown

- Dittos don't stand a Chance against you when you are disguised, you can mock them as much you want they can't use their only Move Transform in the Meantime

- Use Detect first if your Opponent has charged an Attack if you got switched in to avoid all of the Damage.

- Don't make your Disguise get blown too fast when you use Moves your Partners can't learn. Like Nasty Plot or Dark Pulse is a good Combo. At one Point though you can strike with Night Daze

- Anything else you need to know? Well if your a Parnter of you is on a shiny. Then you're disguised as him as a that shiny Variant of his Species.