Jellicent @ Choice Specs
Trait: Water Absorb
EVs: 252 HP / 252 SpA / 4 SpD
Modest Nature
- Water Spout
- Hydro Pump / Scald
- Ice Beam
- Shadow Ball
252+ SpA Choice Specs Jellicent Hydro Pump vs. 252 HP / 252+ SpD Heatran: 306-362 (79.2 - 93.7%) -- guaranteed 2HKO after Leftovers recovery
252+ SpA Choice Specs Jellicent Scald vs. 252 HP / 252+ SpD Heatran: 222-264 (57.5 - 68.3%) -- guaranteed 2HKO after Leftovers recovery
Either move 2HKOes specially defensive Heatran, which can't do squat in return other than Roar it. If it's raining, Hydro Pump OHKOes.
89% 252+ SpA Choice Specs Jellicent Water Spout (133 BP) vs. 252 HP / 252+ SpD Heatran: 372-438 (96.3 - 113.4%) -- 75% chance to OHKO
Also take note that unless Lava Plume crits, Jellicent has a chance to ohko it with Water Spout provided it hasn't been weakened.
Specs Jellicent is meant to be used in rain though, where it can fool opponents into thinking it's running a defensive set, only to be smashed by a poweful choice specs-boosted water-type attack.