Endeavor and U-turn are the best fourth moves on the AcroBling set. Acrobatics, Rock move, and EQ are givens, but I've found that coverage moves like Dragon Claw and Crunch are nigh useless because neutral Acrobatics hits harder in basically every case. U-turn if you're cautious, Endeavor if you want to have that last ditch just in case.
In all honestly, Defeatist has rarely been a problem for me when using Archeops as a late game cleaner. He's such a glass cannon as it is. He rarely gets hit, and when he does he usually goes down, so the lousy ability is a moot point 90% of the time. Besides, he can still do SOME damage with his high powered STABs even after being crippled, and Endeavor can catch people off-guard.
Also, my two cents on the Rock STAB: I prefer Rock Slide. Archeops can't afford to miss with Stone Edge, and as droolworthy as Head Smash's damage output is I don't find it worth the massive recoil (personal preference). Plus flinchhax are always welcome.
Archeops is plenty capable, just avoid priority like the plague and spin whenever possible.