You can't avoid accidentally leaving DoTs up on Ifrit going into the last nail phase sometimes BUT you can control the nail deaths so you don't get multiple explosions even with 2 SMNs as long as they're not doing their own thing instead of following the other 2 DPS. Even with one SMN I've seen them DoTing nails across the room when they should be DoTing the nail everyone else is on and Baning to the surrounding nails. Fester should be dropped on the currently focused nail and the other two single target DPS should easily keep the nails falling in order without the DoTs accidentally detonating an extra nail.