eth obedience, gully face, duriels shell -> might merc
you have a point in life tab because you skilled lower res
so just watch the health of dude man and just cast life tap in between to top him off.
that's the most budget I could think of
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more optimal way to kill bosses with a poison necro is having points in skellies, wearing bramble armor (yourself, not the merc!) and you cast amp dam. You can look into it, when they buffed thorns some time ago there was a bunch of videos (maybe micro bio boi had a good one?) explaining how to best play summon necro now. Bramble is very good chest for poison necro because of the damage multiplier for poison as well... I tried it and found it's a pretty good synergy. In that case merc should also hit hard and have CB on gear.
will it compete with hammerding, grief smiter, warlock etc.? Nope... but poison necro just isn't a good boss killer.
D2:R