How to make an Illusioner do extra damage
Solution 1:
There may be a bug in the game, I am not sure though.
You can however give the illusioner an enchanted bow.
/summon minecraft:illusioner ~ ~ ~ {HandItems:[{Count:1,id:"minecraft:bow",tag:{Enchantments:[{id:"minecraft:power",lvl:10s}]}}],Attributes:[{Name:"generic.maxHealth",Base:100}],Health:100.0f}