I don't think any boss in X5 can actually be classed as being hard, or even remotely challenging.
Agreed. I don't even remember X5 and 6 bosses. I just know they were light years easier than their stages. Which says a lot since their stages were pretty easy to begin with.
Hardest boss for me has to have been Armored Armadillo. Simple move set, easily predictable, yet I get my ass kicked by him without that shockypowerthingy. Every. Single. Time.