The end-of-round text Llamaman found about the maw shrinking if it doesn't envelop you suggests that the way to beat it is to dodge until it shrinks away. Looks like with the other test monsters you get one technique for beating their ultimate form and a different one for beating them otherwise. So presumably Hoop Jump is the regular technique and the top-tier one would be for never getting enveloped.
The question is: how do you avoid getting enveloped? There are plenty of available chain 4 and 5 evasion techniques, but there's only one in chain 3 (Parry) and one in chain 6 (Defensive Burn). It'd be good to know if Parry works vs. the Envelop attack. Since Defensive Burn is unearthly, presumably Kinak didn't make it so that it'd be necessary to get the top-tier technique. The base form is listed as having more than two attacks—does it only use two or fewer each round? If not, is there some predictable sequence to its Envelops? Or do you just have to get really lucky with draws and timing to shrink it down?
EDIT: Cryptodynamic tried this and got the Rasping Teeth technique after five rounds. He said it sometimes used chains of three techniques, but only when it had done the envelop attack as its second technique. So it looks like if you don't get screwed with your technique draws, just chain 4 and 5 evasion techniques ought to do the trick.