By trade.
First, you have to get the specific fossil from a backpacker in the Ruins that corresponds to Archen. Take that fossil to one of the counter clerks in the museum in Nacrene City. Keep leveling up the Archen until it evolves into an Archeops.
You can't you have to go to relic castle and on the west side of the B1F there is a pokemon explorer and she gives you whichever fossil you choose, choose the plume fossil and restore it a nacrene gym and out will come archen, you have to evolve archen to get archeops
Relic Castle
Tornados if you like the legends Maybe silyipyh (desert resort). Or Archeops (fossil from backpacker in castle) Archeops has EPIC stats
Relic Castle
Desert Resort.
you don't "pass" relic castle you go to it later and meet team plasma (you also get a fossil if you talk to the backpacker!)
if you go down to the very bottom of the relic castle go up the all the stairs and you should be out. hope this helps! =)
there is a backpacker in relic castle who will give you either a plume fossil (which is revived into an archen) or a cover fossil (which is revived into a tirtouga)
relic castle
at the relic castle
at the end of it