Have killed four of them...300wm with 180 partition (two shots at 200 yards, second through both shoulder blades dropped him), 30-06 with 180 partition, then two with 350RM and 250 hornady. The '06 bull died soonest, one shot tight in the crease at 30 yards, mushroom under hide opposite. Each 350RM bull took the coup de grace behind the ear, but they weren't going anywhere.
Watched a bull take two unfortunately chosen 165gr SSTs out of a 300 RCM in the chest, then a 150 NBT out of a 270 right in the neck. It was as if the bull ignored all 3 and sauntered slowly off into the woods. A few minutes later the SST shooter dropped the unhappy hang-dog bull in the woods with two quick ones out of his glock 20...the high shoulder shot jiggled spine. I finished that one with a 44 mag and 310 lee hardcast to the base of the skull. If that sounds like a bit of a shitshow, well, it was, come to think about it. But it was never out of control, the packout was all of 200 yards to the river, and it sure is nice to have help cutting them up!
They can really soak up some lead, and I'm not sure there's a magic formula. Just a big animal.