Merc will often get popped from a stack of hydras or curse, even poison if you’re not paying attention.
Optimal dps:
(1 point redemption for sustain) might aura, eth reapers (sheal) eth fort, Andy(budget ral or 15/30 jewel)
No redemption - eth insight (ca/gpa/gt) same armor/helm
Or if you want cheaper/lazy defensive:
Holy Freeze is probably still the standard but since I love
Payer aura with cure and insight (help merc stay alive/aura stick) and I just run with treachery
Still need to pay attention if you get cursed wait for cleans to clear.