I’ve completed 6 chaos wastes runs in the last 2 days and not seen a single grudge marked minotaur. Any advice on how to find these bastards? The spawn frequency seems way too low.
I think you can only get them in the final arena of an expedition, if it has beastmen.
As @Throne_of_Rats has said, the only chance to encounter Grudge Marked Minotaurs is in the end of expedition arenas as they don’t appear for Chests of Trials.
To add on that: Obviously, you need Beastmen in the final arena. And luckily you can check the enemy combination of the final arena already at the beginning of the expedition. So if you are after this specific challenge just take a peek at the beginning and restart if necessary.