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252 SpA Raikou Hidden Power Ice vs. 0 HP / 4 SpD Sceptile: 276-328 (98.2 - 116.7%) -- 87.5% chance to OHKO
You forgot the dragon typing.Mega Sceptile can beat Raikou if it gets a Lightning Rod boost or 2HKO with Giga Drain is SR is up (HP Ice can't OHKO without boosts), but bumping Talonflame's Speed EVs to 168 to outspeed it might still be a good idea.
EDIT: Forgot about Assault Vest sets, but Giga Drain's healing can let you beat it if SR is up on Raikou's side.
252 SpA Sceptile Giga Drain vs. 0 HP / 0 SpD Assault Vest Raikou: 90-106 (28 - 33%) -- 92.2% chance to 3HKO after Stealth Rock
252 SpA Raikou Hidden Power Ice vs. 0 HP / 0 SpD Sceptile: 138-164 (49.1 - 58.3%) -- 97.7% chance to 2HKO
You also check Scarf/Specs locked into anything other than HP Ice and can beat CM if you get a Sub up while it boosts, but it's unlikely.
252 SpA Raikou Hidden Power Ice vs. 0 HP / 4 SpD Sceptile: 276-328 (98.2 - 116.7%) -- 87.5% chance to OHKO