not really, but cleric and warrior should be lvl 50.
Mercenary can build up power by not hitting, by equipment and even more by choosing "charge up" skill, over several turns. (so "counter" or "defense formation" is fine - counters then sometimes do 5 or even 7 k damage and do NOT consume the built-up power)
2nd turn or later: start "infusing" with heat, chill, wind (cleric, lvl 50) and weight (warrior, lvl 50), then choose Mercenary: Supersmash and Ivel: Agony
boom: 20k or 30k or 40k or so damage.
all 4 infusions together make special infusion (lasts only 2 turns iirc), you can put more infusions on top of it.
wind and weight infuse to everyone, heat and chill only to Mercenary.
you can choose some buff skills pre-battle (magnify, boost (cleric), fighting spirit (warrior, higher lvl dog) and determination (Mercenary, warrior) and then restore TP and or mana by items before battle.
equipment here was :
Mercenary: best bat and bracer, counter+20, overload charger and to compensate the negative: HPR5%. Ninja tools, and if you have some: dragon scales (horned lizard, rare drop)
Ivel: magic bracer++ (NOT the magic 2x but HP 1/2 thing), infusion crit amp
cleric: the church skill amplifier, either best hammer or seven-branched sword (better but not necessary, her healing is AT-based)
warrior: trident or naginata+, Ittou, not really relevant
after beating the pontiff, before approaching the sorcerers, you can get to the church from underground, there are some goodies to get. and maybe do some other things if you need more level - mainly warrior and cleric >= 50