Supposedly it's tuned for R5, if you believe the notes in game. I think it's reasonable that some would be doable with significantly less gear, but the best characters and good strategy (like this one). I did it first try with R2 DR/DBast, R1 Malak, G12+ EP/Vader (EP lead of course).
On the other hand, I think it's fair that some really do need more gear. The Ewok tier 2, for example, was near impossible with just a couple R5s (such as B1/B2). But if there was R5 Magna also, it became doable. Same with Rebels - very hard with G12 BH + low relic Bossk. But a squad that was all low relic could reasonably handle it.
