Okay, I have got a problem; I have recently thought about getting a typical Mixpert, but with Counter or Mirror Coat. However, quite by chance, in trying to obtain one, I managed to get one with flawless IVs in Atk / Spatk / HP and then fairly decent ones in Spdef (and a crap Def, but wth) and I now want to try and use this one.

So, here is the set I already have:

Swampert@???
Quiet Nature (I have a plan with this)
252 HP / 76 Spdef / 80 Spatk / 100 Atk

Ice Beam
Surf
EQ
??????

This particular Swampert, with the EVs listed, can survive a Draco Meteor from a SpecsMence. Similarly, having survived said blast, can OHKO a Min SpDef / HP Salamence (which most SpecsMence are) with Ice Beam thanks to the nature and EVs. (It has some margin of overlap but that's not a problem. )

Surf and Ice Beam, hitting from the special side, can damage defensive threats while EQ takes out Spdefers.

However, this means I am completely screwed by the likes of Cresselia and Uxie/Mespirit. I need a fourth move that DOESN'T require breeding, and does something useful on the set.

Also, an item would help, and Lefties is sort of out of the question (as my Skarm carries that and I always play by EVERY clause so I don't have to rethink my team when someone uses an annoying clause that I hadn't considered.)

You could use Roar,so you can phaze those threats away...
Stealth Rock,Stone Edge so you can get the 4x weak against...
Or you could go for a Rest/Chesto...
Or Focus Sash/Endeavor what would take something down if you have Sandstorm on the field...

Or you could use him(or her) to breed and keep the good Ivs...
And get Mirror Coat/Counter...

I'd use Roar as well to phaze out those Cresselias and Mespirit. They both can utilize Calm Mind so roaring them can mess them up badly. Besides, Cresselia can't do much to Swampert, but I'm not sure about Mespirit's Psychic. This, obviously, goes well with Stealth Rock/Spikes support from Skarmory. Rest + Chesto is the way to go with Item Clause active, however.

I agree with everything said here. If playing by all the clause's I'd use the Rest/Chesto probably. Otherwise Roar is a solid choice.