Without Trick Room, because I normally don't run that, means it'll be more of a defensive poke for me, if not already. No recovery moves and/or being a mixed attacker is sure death it seems. You can Rest probably but that'll just leave you open. For support, I'd rather have Kefki for screens + entry setup, and if I'm going for straight stall/bulk; Shuckle has the best advantage for that, he has the highest in the game, he's got it covered. Diancie's x4 weakness to Steel may be a problem due to Aegis/Aggron, or even x2 from an EQ. Everyone has a fariy killer prepared. EQ is just powerful. 50 base speed? Maybe Trick Room is the only way since Rock Polish won't exactly help. Though you said the offensive move pool isn't anything to be proud of and it's split. I'll have to wait and see for the Mega.
Which would you choose if it were offensive, Physical or Special?
Trick Room Support / Stealth Rock Setup
Diancie @ Babiri Berry
Ability: Clear Body
EVs: 200 HP / 188 Def / 120 SpD
IVs: 31 HP / 31 Atk / 31 Def / 31 SpA / 31 SpD / 0 Spe
- Diamond Storm
- Stealth Rock
- Trick Room
Berry is for 1/2 damage from Steel (x2 instead of x4) so you can setup Trick Room
Stealth Rock setup at least before dying
Attacks are for stab and if Taunted/Tormented/done with setup
Combine this with other Slow/Trick Room pokes or Vs fast OU pokes.
Tested this ~7 times on Showdown, usually won't go very well. But rarely, it's perfect.
I probably have no idea what I'm actually doing, so by all means, someone hand me my ass back.