The reason Biochemists don't really work against those MVPs is that they both have 25-30 VIT, well below the amount required for Acid Demonstration to do its work properly. Ifrit also has access to Pneuma to make things complicated. Unfortunate. D: However you're right, they'd be very helpful as a supportive role, just not as one of the main damage dealers.
A Sage/Scholar helps a lot using Dispel to terminate their buffs (Randgris' Assumptio and Ifrit's Two-Hand Quicken) although you could just as well have a priest lv1 KE Randgris to cancel her Assumptio.
In case it isn't known already, Ghostring Armor helps a lot against these guys, Earthquake is a serious issue, enough that sometimes people just solo Ifrit instead of take a party along.
That's about all I can think of really. @_@ Better luck next time!