This just prove, again, that Codex are not very tactical gamers. I dont even mention shits like minmaxing like a bunch of munchkins, but just pure tactics.
1. Targos is hard simply because you are starting with a new party so you lack options. Ranged attacks or melee, the end. So how to make it easier? Open your wallet for a wand of grease from the elf chick Dinnsmore, that will open up your options. With an arcane caster and a wand of grease you can launch more grease which can stagger the enemy waves and make them easier. And/or Flaming oils from Oswald which make for poor man's fireball. It's about the only time in game where Flaming Oils pay off, as later enemies are too tough to bother with it.
2. The drums are a great source of enemies when you just get two of them a time. And the key to get that is to NOT poke your nose too near the main concentration of hostiles. Get out, and lure them back to your waiting party and kill them. BEFORE you get near the drums. If you play like storyfags, as with most players of BG, you will get your asses holed by having two riders drop on your weakest characters while others are delaying your melee protectors. But if it's your six against two of them: xp+loots.
3. Clerics are pure casters without need of scrolls so need to be as high level as possible. Why would you dilute caster's DC and arsenal by dipping into other classes? Plus they are heavy armor wearer, which should play merry hell with monk's bonus.