so, i did it, more luck than judgement, lots of theories here:
https://forums.galaxy-of-heroes.starwars.ea.com/discussion/261936/tusken-galactic-challange/p1
perhaps if the enemy gives you momentum, that doesn't count, you have to "earn" it yourself?
so, tier 4 I did with all tuskens, not really sure how
last tier
Bossk, boba Sion, Wat, Hoda, Malek
health tech malik, weapon tech boba, tank tech hoda, hoda training on on malik so he gets targetted, then just keep using boba's right most aoe ability, middle ability when you can't use right most
I "think" if boba dies and revives, it resets your count as took a few goes, and go that worked, he didn't die
but as we know, it all seems to be rng related and not like the description, a bit like that ships challenge where the description never gets fixed and has been wrong a year or so?