Got it without symbiosis active.
But, it was after having gained some symbiosis in many fights before this day, even 2 energy of it once,
and after feeding the ooze several times over (at least tice into each form, I think - that also seemed to help with the "2 energy of symbiosis" thing, couldn't get it before even with completely evading all damage from 2+ attacks in 2 passes, got it from evading 1 (I think) just after refeeding. Then lost symbiosis by dropping pants of course.
Hungry Ooze Lash / Discussion
This is the discussion related to the wiki page Hungry Ooze Lash.