Dealing with Ability Drain in Combat

[HMS Invincible]

The problem I'm having is dealing with outsiders or undead who can drain ability damage and convert it into temp hp. With 3 attacks, it can drain 1d4 charisma or 1d4 intelligence which would disable most of our party members in a single round.
This has happened twice already, and I'm not sure what spells to utilize. Should we have just cast restoration repeatedly and focus fire it down? Our average party level is lvl 11, and we had an occult slayer, a fighter, wizard, and archivist. I didn't know how to debuff it properly, so I casted polymorph and called it a day. After the party got defeated, I threw down a freezing cloud, and tried to throw orbs of fire with my blindfold of blindsight to buy time. It planeshifted away before I knew if it was going to work or not.

I don't know the actual monster name, but it has 3 attacks, outsider type, planeshift 1/day, and drains 1d4 int if you fail a save per attack.

[Feralventas]

By level 11, you're going to be dealing with things like ability damage and ability drain on a more regular basis. The higher level you go, the more complicated the conflicts will get. Attacks with level drain, ability damage, or other penalties are something you're going to need to learn to deal with if you want to progress any further unless you can talk your DM into sticking to straight-up damage progression (at which point prepare for fort-saves vs. massive damage).

There are a number of class features, items, and spells of which can immunize yourself or others to ability damage or ability drain. Start looking into these if you want to try to step ahead of the curve on that front.

Additionally, you can try to prevent the strikes from hitting in the first place; invest in armor and items and abilties that will boost your Touch and Incorporeal Touch AC; can't drain what it can't touch.

There're two creatures that I can think of to fit your description, and mitigating either the ability drain or the touch-attacks will do wonders for minimizing your risks. Additionally, look into the Wall of Blades maneuver in the Tome of Battle; your attack roll becomes your AC for an attack, and that's easier to augment than your AC in most cases.

For debuffs, grab some spells that function as natural daylight. Like Daylight. (3rd level sorc/wizard, evocation.) A lot of incorporeal (and corporeal for that matter) undead are utterly powerless in daylight. This might be a bit cruel though, so keep in mind that it might neuter the entire encounter.

[JeminiZero]

You could have the archivist prepare Sheltered Vitality, a L4 spell that provides immunity toability damage/drain. But this is highly situational, and the spell is useless if not fighting ability damage/drain enemies.

Slightly less situational, is to summon something immune to ability damage/drain and have it tank. Wiz/Archvist has summon undead, but that has the evil descriptor. Archvist also has Conjure Ice Beast and Summon Desert Ally for constructs.

[holywhippet]

That assumes the archivist even has those spells - they only get 2 free cleric spells on level up and anything else they can find as a divine scroll.

If the creature has been summoned and is (presumably) evil then protection from evil will prevent it from actually touching you unless it has spell resistance.

[HMS Invincible]

That assumes the archivist even has those spells - they only get 2 free cleric spells on level up and anything else they can find as a divine scroll.

If the creature has been summoned and is (presumably) evil then protection from evil will prevent it from actually touching you unless it has spell resistance.

It has planeshift 1/day so it was not summoned, that I know of. And it has spell resistance, so it doesn't matter regardless. Is there any other spell that protects against ability drain? If not, I'll tell the party to invest in magic items to gain immunities.