This achievement should unlock if you get to Dr. Vendetta and die (this is how I got it)
but since the achievement is "defeat Dr. Vendetta", here's a strategy for him:
the battle sort of, kind of has 3 phases.
1st phase: Vendetta uses his normal attacks
2nd phase: Vendetta begins using an area attack that shoots flames out in X and + patterns, then finishes with an AOE explosion.
3rd phase: Vendetta summons black screenmen (the ones that shoot from a distance)
this fight on hard is SUPER FREAKING ANNOYING. Best tip I can give for hard is to never shoot (unless you're close to beating him), and use your shield A LOT! As you lose to him, you should get a sense of what you can block (his punch combos), dodge (the slapping butt dive (theory: is secretly R. Mika)), and run from (hat toss, AOE flame attacks)
Thankfully there's a trick you can use that can stunlock Vendetta for big combos and decent damage. The irritating bit is luck plays a big role in pulling it off in addition to beating him. Its not a winning strategy, more like evening the odds.
What you want to look for is Dr. Vendetta flinching when being hit. Most of the time, this will allow you to nail him with a whole combo without being interrupted. The chance he'll flinch is very very low hitting him randomly (it does happen sometimes). The chance is very high after you deflect an attack of his. The easiest thing to deflect is his heavy 3 hit flurry combo.
Only use the
Here's the important part: the moment you deliver the combo's final hit and knock Vendetta down, quickly tap
and begin the
combo again. If you did it right, Vendetta will continue to flinch and you will restart your free combo on him. If you're super duper lucky, you can do one long continuous combo going right into him doing his AOE attack.
You can do this with both characters, but I had better results with Mike. He doesn't have a wind-up like Dorian has.
for 2nd phase, keep up the same stun lock combo stopping to get away when he does his AOE attack
3rd phase: the good news is Vendetta will attack less, so keep using the stun lock combo every chance you get. Vendetta will break out of it more frequently, but will normally go into trying to summon screen men. KEEP ON HIM. You can interrupt his summon and keep the screen men from appearing. If Vendetta does manage to summon the screen men, keep on attacking him. It may seem like a bad idea, but you're actually putting yourself more at risk of dying and starting the fight over if you don't. You will take damage from the screen men, but they're not super aggressive. At this point, you want to go nuts with melee on Vendetta since he won't hit back as much.
The stunlock combo isn't a guaranteed thing! Generally, Vendetta will randomly snap out of his stun and begin attacking you. If you're quick, you can re-deflect an attack and attempt the stun lock over again. Typically I found myself getting a chain between 20-30 hits before he truly broke free from the stun. A few (very lucky) times, I managed to get over 80 hits