It looks like you're dealing with a lot of summoned monsters (Summon Monster 7&9, Wooden Phalanx), so you'll want the pit fiend to have a bunch of minions/allies who can intercept those and deal with them.

For Clashing Rocks, you probably want him to have some form of immediate interrupt that lets him get out of the way of it. Although the image of a pit fiend getting buried under a pile of rubble is pretty cool so it's not a big loss if he dies that way.

Implosion - For the minions whose purpose is to keep the summoned monsters busy, it's not a big problem if they get whacked off by this, if anything it's a win because that's a turn the cleric isn't focused on the pit fiend.

Mass Heal - Pit fiends don't really have a way of dealing enough damage to KO someone in one round so this means it will be using Trap the Soul a lot. Quickened fireball could be a way of trying to get the cleric to waste his turn on Mass Heal, though dispelling is still the priority.

Miracle - be ready for what you'll do if the cleric uses its request function to try and win. Don't just say no, have something cool happen just not an auto-win.