I managed to take out the 4 rangers that come upstairs, but had to leave through one of the side doors to avoid the 3 warriors downstairs. There is no need to get the key early, though, and as far as the experience bonus, you get the same amount 3 or more levels below your opponent. With level 28 opponents, for example, taking them out at level 25 or below would give the same experience.
xp points per level list? spoilers possibleFor my strategy, I just summoned my creature (mage head), did a bit of damage with a bow if I had enough cover or they were mostly focused on my creature, Rush Attacked to do damage (duel wielding warrior) and withdrew back upstairs when I needed to heal (potions and/or level 1 of the spell). If only one ranger came after me upstairs I would be ok; 2 alone were a little too much to handle in a confined space, so I would jump over the railing and Rush Attack someone downstairs, until I needed to break away to heal. Once you take out the first one, it gets easier, especially if your creature can keep the attention of 2 that are left for awhile.
With the 3 warriors downstairs, they Rush Attacked too much to be able to handle at my level, given the time it took to heal and recover if I was stunned.
On normal difficulty I was level 16 or 17 when I took out the first 2 Black Ring warriors, but couldn't handle the rangers upstairs. When I tried again about level 21 or 22, the rangers were not too bad, but I had to skip the warriors. After getting the battle tower I left the return to Broken Valley fairly late, so had no trouble taking them out in the early thirties for level.