I'm running an arena-style fight between a lot of PCs. It's now getting to the point that there are few enough PCs that they are beginning to take levels to counter one another.
One of the players is a Rogue. At lvl 11, the die roll for ability checks rounds up to 10 when they are proficient - for a rogue with a dexterity of 20 and expertise in Stealth, the minimum Stealth check is 23. Two levels later at 13, it rises to 25, and eventually to 27 at lvl 17.
The first situation is difficult enough for opposing characters, but by lvl 17, only Perception-proficient characters who have good Wisdom even have a chance of spotting the Rogue.
I know some of the casters have their own advantages in class features (Hold Person is pretty lethal), but those all require a save at least. The rogue can shoot from hiding every turn and re-hide, dealing several dozen damage and being near-impossibe to find.
Besides staying in open areas without places to hide, what could any other PCs do to have a chance at winning?
Both numerical and strategic considerations are appreciated. No UA or homebrew is allowed as a source, for obvious reasons.