This worked for me (2 cleric mercs and 1 dps caster merc): Keep one healer merc on passive. Kepp the dps merc on burn (he will son be out of mana, so better burn the named down).
One the active healer is lom, bring the second to active.
DPS as much as you can.
If you have a DOT class, go for full dots, right away.