I entered this into a damage calculator:
252Atk Life Orb Garchomp (Neutral) Brick Break vs 252HP/252Def Leftovers Blissey (+Def): 53% - 63% (384 - 454 HP). Guaranteed 2HKO.
This is assuming max IVs, full HP and Def IV and EV investment in Blissey, neutral Garchomp Attack with max EVs and IVs along with Life Orb.
No STAB, so 2HKO. If you had Swords Dance, you can obv. get up to 1HKO.
Now Blissey VS. Garchomp with Ice Beam, that's interesting:
252SpAtk Blissey (Neutral) Ice Beam vs 0HP/0SpDef Garchomp (Neutral): 92% - 109% (332 - 392 HP). Guaranteed 2HKO. 58% chance to OHKO.
This is providing that you invest 252 EVs in Sp. Atk though. Blissey Sweep!