Grimgor absolutely did this lol, it’s pretty explicit that he alone fought dozens of throts strongest rat ogres at once. His army was around, but he took the ogres on alone. U5 are not anywhere near people like grimgor or other named lords, they’re very strong but we’re fighting guerilla operations against mostly just slaverats and clanrats, they’re not facing down major clans or anything like that.
Grimgors about the same level as Crom, who would absolutely massacre anything we’ve faced. Like Bodvarr is a notable challenge to the U5, but people like Crom or Grimgor would just dismantle him with no real issue by themselves. Even if you count mods, the U5 are nowhere near the strongest characters in warhammer.
That tier is full of people like nagash, teclis, literally sigmar himself, archaon, tyrion, malekith, all sorts of absurdly powerful people who wouldn’t even bother fighting the U5 they’re that far below them.