DEBUFFS. DEBUFFS. DEBUFFS. MIASMA HAS DEBUFFS THAT BOSSES ARE IMMUNE TO.
Even the ones you can inflict like blind have extremely harsh diminishing returns, making a second summoner obsolete in debuff application.
P.S. Spam Ruin II without bard popping mana song, please. Let's see how many festers instead of energy drains you can use then.