Just idle curiosity from a longtime lurker. I've found some old threads along the same lines, but wanted to take into account new published Paizo materials.
I think I came up with a decent contender. 20 Magus (Bladebound / Kensai). Check my math for mistakes.
The magus is minding his own business, walking down a road, fully buffed. A bored deity summons the Tarrasque right next to him. Surprise round!
With his kensai training, there's no doubt who goes first. Weapon drawn and already sporting his enchanted katana, the kensai makes a single spellstrike attack with an empowered and maximized Disintegrate, catching the Tarrasque flat-footed.
For the sake of the story, the attack hits, the mighty beast rolls a 1 to fail the Disintegrate save DC and the attack is a confirmed critical hit.
The magus has a x3 multiplier with his katana from his capstone feature and casts Decapitate as an immediate action, raising the critical multiplier by 1, to x4. He also spends 2 points from his arcane pool to increase the multiplier again, to x5.
With all that so conveniently lined up, I think the Tarrasque's head is flying off with a massive +1,000 HP slice from his Black Blade.
Without spelling out the whole build, assume the Magus has, with items, 40 Intelligence, 26 Strength and all the relevant feats. To summarize the math: an average ~300 slashing damage (1d8 +8 Strength +5 enhancement + 6 Black Blade Strike + 6 Power Attack +2 Bane +15 Intelligence from Iaijutsi Focus) x5 +4d6 from Decapitate +2d6 from a Bane weapon +720[(+60d6)x2] maximized and empowered disintegrate + 2d10 electric from Shocking Burst + 2d6 bleed from bleeding critical + stun from stunning critical just for good measure.
If the Bane weapon doesn't count as Epic to over come DR, subtract 15 damage!
Maybe the Tarrasque isn't the best example, but it makes for a more compelling story than having a kensai 1-shot a rabbit he passed on the road.
Anyone have other suggestions? Anything conspicuously awry with my calculations? Only thing I wasn't sure about was whether Decapitate's multiplier boost would stack with the kensai's Perfect Strike boost.