I don't think that's true. A whole lot of bosses are harder than SCal, they just need less gear to defeat and you have the ability to give yourself an advantage over them. The Old Duke, for example. Normally, you have the freedom to get better gear than required before you try to take it down, but in the boss rush, it has significantly more HP and attack damage, and I have never been able to get past it even with full endgame gear.But boss rush is really trivial with endgame gear? I mean, boss rush in its entirety is easier than SCal Even SCal, the only boss in Boss Rush that could potentially kill you, is capped at 3million HP instead of 8million.