All Pokemon that can use Frost Breath as it is (i.e., not via Nature Power) are either pure or part Ice types, with the exception of Mew, meaning all but Mew will receive STAB on Frost Breath.
For this I'll exclude Nature Power (because in wifi battles it can only turn into Tri-Attack).
Bermite is the one with the lowest base sp. atk at 32. Its minimum at L100 is 62, so even with STAB:
>0 SpA Bergmite Frost Breath vs. 0 HP / 0 SpD Krookodile on a critical hit: 78-92 (23.5 - 27.7%) -- 71.9% chance to 4HKO
This is with 0 IVs/EVs in Sp. Atk. Krookodile's stats are all at 31 IVs and 0 EVs.
If you exclude NFE Pokemon, Avalugg has the lowest sp. atk, at base 44. Its lowest sp. atk at L100 is 83, so:
>0 SpA Avalugg Frost Breath vs. 0 HP / 0 SpD Krookodile on a critical hit: 104-126 (31.4 - 38%) -- 89.1% chance to 3HKO
Again with 0 EVs/IVs in sp. atk.
HOWEVER. Avalugg is incredibly slow, with an even lower speed stat than sp. atk. Krookodile needs to survive not only two of your opponent's hits, but also Avalugg's Frost Breath crit before it can get the Anger Point boost, making this strategy super not viable...
If you really want to do this, try Mankey/ Tauros because they're not weak to Frost Breath.