Almost anything with sucker punch and good enough attack, anybody which stabs shadow sneak with a good enough attack, any strong unresisted physical priority, faster mons with strong physical stabs/supereffecive moves, and bulky mons with strong physical stabs/supereffective moves which don't take SE damage from Mega Alakazam.
Good sucker punches include (mega)absol, cacturne, toxicroak, and most prominently Mega Kangaskhan (breaks the sub, in case he tries to play smart).
STAB Shadow sneak is an easy feat for physical protean greninja and aegislash (surely the latter, maybe not the former but I doubt it'd miss the OHKO), to name a few.
Strong enough physical priorities include swords dance (mega)scizor bullet punch (cb scizor has "only" a 50% chance to OHKO, and with entry hazard pseudo-immunity through magic guard, that requires weather damage after the megaevo to nab the KO for sure), belly drum azumarill aqua jet (or PuP splash plate at +2), gale wings talonflame flying move of choice (most likely brave bird), and many other strong physical mons with life orb/choice band and a priority, assuming they got a +2 atk baton passed.
Faster mons include only a couple scarfed threats, assuming there's no tailwind/thunder wave support: adamant scarf terrakion stone edge/x-scissor, and adamant choice scarf garchomp earthquake/outrage. Other scarfed threats are not fast enough or powerful enough to manage the OHKO.
For bulky mons I don't feel like running calcs, but maybe a standard choice azumarill can do it with waterfall, just to name a possible option.
This is assuming singles.
In doubles, the options increase, as many more mons can easily nab the OHKO with a physical priority assuming a helping hand/self swagger boost, or just some little previous damage courtesy of a fake out or whatever.
You can also neuter its speed with a bulky mon which can set up a trick room for you and then makes way for a slow, powerful trick room sweeper to do its work; this should be doable in doubles, most likely.
Or, instead of trick room, you could set up tailwind/thunderwave it and laugh at it.
Another, gimmicky, way is to just use a focus sash/sturdy mon which can ko it with a physical move after surviving the first hit somehow. This includes F.E.A.R. Rattata and Aron.