It's going to be pretty hard running Heatran, since it's slower than P-Groudon and P-Kyogre (maybe P-Kyogre, since P-Groudon usually runs low speed), which both can OHKO it. So chances are, Heatran will be a rare scene. Thundurus won't be that common either, as some Cresselia bring Gravity (with Mental Herb, to prevent Taunt), so Precipice Blades will land and OHKO Thundurus. Landorus will probably be present (I mean, Intimidate is even better now with Competitive/Defiant Pokemon not being common nowadays), although I believe a Special Attack Lando-T would be best due to having access to Earth Power (no Wide Guard protection) and Grass Knot (for P-Kyogre).
I'm interested in Dialga though. I haven't battled one ever, so I don't know how it works. But I'm kind of not impressed by this:
252+ Atk Primal Groudon Precipice Blades vs. 252 HP / 0 Def Dialga: 186-218 (89.8 - 105.3%) -- 31.3% chance to OHKO
Is it supposed to be a counter to Mega Rayquaza and Xerneas?