I would choose a 450NE or 470 or 500 without much preference between them. But when I was close to elephants I would be second guessing my choice if it wasn't the 500.

If the lion is at 100yds, I would hand the tracker my DR and take from him my scoped 375H&H. Prior to approaching the downed lion I would clear the chamber of the 375H&H, replace the spent round in the magazine, and give the 375H&H to the tracker while taking back my DR.

JPK