Need help figuring out how to make my Merc spam Vaal Vitality, or use it, at least sometimes.

Found this nice setup but my guy almost never loses any hp, or just dies and I find out his dead after map is over. Kinda crazy how these 2h guys have 2k regen without much effort... Anyway, he never casts Vaal Vitality. The only way I found him use it is in Sarn arena a guy helped me get him to low hp when he casted it. Maybe I can get him to low hp somehow? If he was Int I could use 2 Midnight Bargains to reserve his hp to 40% but I can't see anything that could help a str guy. Any ideas? Preferably smth that doesn't kill my duder.