Drezen is pretty cool, but its easy to get lost and there's a lot of difficult encounters. I think the hardest so far are Shadow Votaries, who can trap your entire party before you have time to react and then just fireball you to death.
It doesn't help that RTwP mode makes your party members suicidal idiots who walks into danger.
There's one particularly nasty ambush in the temple where a Vorock Votary casts Blade Wall from outside of vision range, which willl one shot your characters. That's kind of bullshit.
Use your map a lot to stay oriented. Think of the wall as your hallway to get to the various areas (rooms) and rest after you've cleared each one (there are a lot of the supply stations that let you do so). Clearing top of wall (and gatehouses) first is a good idea anyway since it will eliminate the Giants pelting you. You can get up on top of it just NE of the Temple entrance (and even avoid that fight with good skills if you want to).
Use a Scout (character with high Stealth and Perception) to make sure the enemy is reacting to you instead of vice versa. There's one custom made for that in the Prison.
Shadow Votaries can be zerged with Charges in RTwP (which is a good idea since they seem to have Mirror Image on contingency after the Surprise Round). Votaries are Human so not Immune to Poison (including Stinking Cloud), and can be stunned with Sound Burst (which doesn't target). The Agony Hex that Ember gets at lvl 10 has 60 ft range so is good vs the human casters that seem to be giving you trouble, but only if you see them before they see you.