Archeops is actually not rubbish in competitive play. Even with Stealth Rock Weakness, Defeatist won't activate at full health. It has a base 110 speed which means it can outspeed most things in UU, including Mienshao, Virizion, Victini & Zapdos. It has a really good 140 base attack, and access to STAB Flying Gem Acrobatics. Even with Defeatist, as long as you provide some good support, Archeops is a really dangerous sweeper.
In game, it's not useful since you only get Archen at Nacrene City, post E-4 (B2W2). However, if you traded for one, it would do well at Roxie's gym (Poison resistance), Burgh's Gym, Clay's Gym & Skyla's Gym. Archeops also learns Dragon Claw at level 56, so if you're at that level when you face Drayden it'll be useful as well.
Archeops doesn't really have that much type advantage over the E4, except Marshall, though his Pokemon usually carry Rock moves. Caitlin & Shantal are weak to Crunch (level 35), but you get Zorua for free at Driftveil, so you might as well use that.
If you want to use one as a HM Slave, my advice would be... don't. Archeops has some pretty great offensive moves, and Fly & Cut both waste a move slot.
In game set:
Arcehops @ Flying Gem/ no item
Ability: Defeatist
- Acrobatics
- Rock Slide/ Stone Edge
- Earthquake/ Crunch
- U-Turn/ Dragon Claw
You can consider Roost as well, but that needs to be MT'd
Competitive set:
Arcehops @ Flying Gem
Ability: Defeatist
EVs: 252 Atk, 252 Speed, 4 HP
Nature: Adamant/ Jolly
- Acrobatics
- Stone Edge
- Earthquake
- U-Turn
