These will all OHKO Wailord ex, according to RS-on Modified. This list does not include attacks that do x damage times Energy attached to anyone.
Breloom (R/S), Battle Blast (6+ Fighting required)
Sceptile (R/S), Tail Rap (2 heads needed)
Kecleon (Sandstorm), Double Scratch (basic Grass/Lightning required, 2 heads needed)
Raichu ex (Sandstorm), Mega Thunderbolt (solid 240)
Muk ex (Dragon), Slimy Water (+1 Retreat Cost to Wailord ex needed)
Sceptile ex (TMTA), Slashing Strike (solid 200)
Electrode (HL), Mass Destruction (auto-KO)
Lanturn (HL), Lightning Strike (Strength Charm required, discard all Lightning)
Beedrill (FRLG), Link Needle (3+ Beedrill required)
Exeggutor (FRLG), Big Eggsplosion (5+ Energy required, 5 heads needed)
Charizard ex (FRLG), Burn Down (solid 200)
Electrode ex (FRLG), Crush and Burn (5 discarded needed)
Venusaur ex (FRLG), Solarbeam (Strength Charm required)
Dark Muk (TRR), Slimy Water (+3 Retreat Cost to Wailord ex needed)
Dark Ariados (TRR), Breaking Impact (solid 200)
Beautifly (Deoxys), Cutting Wind (Hunch activated and Strength Charm required)
Manectric (Deoxys), Magnetic Tackle (6 Lightning Energy required and additional damage invoked)
Crobat ex (Deoxys), Pester (Special Condition affecting Wailord ex required)
Cacturne ex (Emerald), Spike Rend (6+ damage counters on Wailord ex required)
Bellossom (UF), Full Bloom (3 Bellossom required)
Meganium (UF), Bouncy Move (+5 damage counters required)
Feraligatr ex (UF), Sore Spot (7+ damage counters on Wailord ex required)
Lugia ex (UF), Elemental Blast (200 damage)
Metagross DS (DS), Crush and Burn (5 discarded needed)
Jolteon ex (DS), Second Bite (6+ damage counters on Wailord ex needed)
Magneton (LM), Multiple Force (React Energy attached and 7+ Magnemite and/or Magneton in play)