Summary/TL:DR: Diplomat (might be possible with Squire or Jack(…)). Go at night. Enter through the stables (front entrance is risky, since you might end up getting too close to a guard). Don’t wear weapons (torch seems fine). Don’t get close to the guards (whenever I get about 1-2m away from a guard they stop and then immediately attack me (tested during the day, they seem to bother less during the night). If you chose diplomat and one of the guard initiates a dialogue, use the speech option.
Diplomat seems to help a lot (as Squire, I’m finding it nearly impossible). I tried several times during the day (once with low conspicuousness and another with high conspicuousness, it seemed to work better with a high conspicuity and as much Cuman armour as possible. I was able to scout around the camp during the day, however if i got close to a guard they would attack instantly.
The second attempt I made partly during the day, infiltrating the camp from the north side (where there is a gap in the stables). I was able to loot 3 of the chests, though I kept a large distance between any guards.
I came back with vials of poison during the night (infiltrating from the stables), and kept about 4m minimum from each guard. And was not stopped while pouring poison into the first pot.
When I got the the pot at the end of the Cuman camp alongside the arrows, one of the more heavily armoured guards interrupted me twice (once while trying to poison the food, and again by the same guard while burning the arrows). I chose to use the speech option, and succeeded on both. Then walked out the way I came.
I also only wore a torch (no sword or bow) and use it to see where the guards were.
Hopefully that helps. I haven’t been able to get past the Bohemian guards up the hill (maybe the Cumans and Bandits agree to keep away from each other), so I can’t help there.