If you want to live dangerously, just let one mob be higher than the other, but then take them into the next fight.
This typically happens when you have a mob of (say) level 55 and a mob of (say) level 51, and so typically as long as mob #3 is 51-53, your already-damaged level 55 mob will do just fine.
But if you run out of mana, or if mob #3 is also 55 ... well like I said, this strat is for living dangerously, and you need to always save some gate mana
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If you want to be more responsible, you need to use damage shields, DoTs, and nukes in the first fight as soon as you see your pet losing, and if your pet is winning you need spam back/sit (or change pets).