Whenever an XP-valuable monster is slain in Hexx Ruins, a Revenant of equal level rises in its place. Revenants grant no XP and have a poison attack. As well, whenever any monster dies, every Revenant will suddenly move in to surround you. If you don't kill the Revenants, they will quickly swarm you. Monsters killed within subdungeons do not rise as Revenants, nor do the bosses.
The main boss of Hexx Ruins, Stheno, has the abililty to absorb strength from the other monsters in the dungeon. When you first attack Stheno, every other monster will disappear and be converted into physical and magic resist. This also includes Revenants, but not monsters that reside in subdungeons, nor plants. There is also another boss chosen randomly. This second boss is not removed from play when Stheno absorbs monsters, but seeing as you are capable of using level-up catapults against this second boss it almost always preferable to leave Stheno for last.
Your top priority is to avoid being surrounded by Revenants. These monsters are not overwhelmingly strong as individuals, but their poison attack means they can exhaust you as you fight and in large numbers they can pin you down and overwhelm you. If you have the HALPMEH glyph, you can try to pick the Revenants off one at a time and then use your magic to cure the poison. You can also use the IMAWAL glyph to turn them to stone. They're not worth experience points anyways, so it's no loss to you to brick 'em. However, IMAWAL can also cut off your avenues of escape, and may increase your chances of being trapped by Revenants in the future. To complement this, you can choose to worship the Earthmother. This yields a free IMAWAL glyph and 10 piety per IMAWAL use, even on Revenants.
The best way to stay safe from Revenants is to avoid creating too many to begin with. If you fight one or two powerful monsters, you will create one or two powerful Revenants. Although these will be dangerous opponents, there will not be enough of them at any given time to trap you. A swarm of 6 or 7 weaker Revenants is a much greater threat. By focusing on killing stronger monsters, you can level up very quickly, and by the time you've created a large number of Revenant you should have already gained enough levels to kill the weaker ones in a single blow.
Once you've figured out how you're going to deal with Revenants, your next problem is Stheno. If there are any monsters left on the level, Stheno will grow stronger when you begin the boss battle. Clearing out the entire dungeon is critical. IMAWAL is an effective way to remove any stragglers you don't want to bother fighting. It's not like you can get a mid-boss level up anyway. The best answer to this is brute force; find a combination of items, glyphs, and deities to make your character as powerful as possible a plough through those monsters, and then the boss.
One tricky strategy that can be used against Stheno is to employ many one-time bonuses to 1-hit-KO the boss. Since the first attack goes through before she absorbs the other monsters, this attack will not be affected by those extra resistances. However, pulling this off is usually more difficult than more conventional approaches.
He begins with the HALPMEH glyph, and the dungeon is almost exclusively physical damage. There's no more straightforward approach than this. The Human and Orc races are suggested for the Paladin.
With first strike and high damage, Rogues have an easier time getting 1-hit kills against Revenants. The difficulty will be improving their durability so they can survive more powerful enemies. The human race is recommended; you will need to find some way to boost your hit points (the the "Absolution" boon of the Glowing Guardian, "Vine Form" boon of The Earthmother and "Boost Health" boon of Jehora Jeheyu all work well for the Rogue here)
Using CYDSTEPP, a Warlord can continue fighting effectively even while poisoned and at 1 hit point, allowing him to kill Revenants without leaving himself helpless. The Orc and Gnome races are recommended for Warlords.
Revenants are not bloodless monsters (unlike most no-XP enemies), and will leave blood pools that the bloodmage can absorb when they die. This effectively doubles the bloodmage's normal resource pool.