I've fought 2 espeons in gyms so far and he hits really hard. They were sitting at the bottom and found Psybeam as his best defensive charge attack (comes fast and more often than FS or P). The other 1 had Futuresight, it hits also super hard but I found it easier to dodge.
As an attacker he can do the job pretty well.
I guess you want these movesets (only my personal preferences):
Offense: ZH/FS
Defense: C/PB