I dunno I kind of think DS TB and LS TB are pretty comparable in difficulty. I'm F2P and started in January 2017, and went 81/84 on DS TB combat. 2 of the 3 fails were on the phase 3 BH mission because my BHs stink. Like people have said, if you're getting creamed with g12 characters you're probably not doing it right.
my squad comps:
zveers, starck, snow trooper, death trooper, storm strooper - 4/4 all phases + IPD mission
zzthrawn, TFP, tarkin, shore trooper, FOTP (just because he's g11...another empire would be better) - 4/4 all phases
EP (no zeta yet), vader, nihilus, dooku, sidious - 3/4 in phase 6, 4/4 other phases. EP lead zeta coming next so hopefully will ace phase 6 next time.
https://swgoh.gg/u/macattack/