I have some ideas that might help Ifrit to be a better addition to the team in the long run.

So fester is only affected by Bio, Bio 2, and Miasma, so, the first thing I would do is reduce the cooldown on enkindle, if that means lower the potency for the Egis etc., so be it. The next thing I would do would be to allow Ifrit's Inferno damage over time effect to work with fester as well, this would give you an extra burst of damage when using ifrit every now and then, depending on how much the cooldown reduction on enkindle would be, which I'm hoping would make him more useful, at least for your regular enemies.