Depends on what difficulty.I defeated him on like 3rd try in challenging mode as hydraulic.All you need to do is to get Restore skill that dispels poison debuffs from you and keep yourself and Veradux alive , until you get a moment to kill that guy.
It took me a long time too; my hint is to basically keep Veradux on the offense and force him to heal himself only when he needs to. I remember my guy absolutely failed at attacking the experiment, and my partner's attack was the only one that dealt any damage.