Actually, water DOES burn when the temperature is high enough. After all, it's made from hydrogen and oxygen, both of them highly susceptible to fire. Pure oxygen, I mean. That's why there are no water extinguishers (I think). They use dust and strange chemical substances.
But honestly, I don't think our Cleric can provide so high temperatures.
Heresiarch isn't so hard. Use Berserker mode and smash him with quietus. (I did so on Berserker level). If you run out of rage, use quartz flask. Don't attack him when he's invulnerable (pink rings around him), or he'll bounce your missiles (the same in pure hexen). The warrior has easiest time with Heresiarch of all 3 classes in KMod. No offence, do you use mouse ? After some practice he's just annoying because you can't attack him all the time. The mana weapons suck for warrior in KMod, they're not worth the effort. I mean, excluding the axe before you find Quietus. Besides, you deal far more damage with (berserked or not) quietus.
Anyway, you only need to kill the initial Heresiarch. Dark Servants have little to do with heresiarch spawning, they are simply waste of time. Additional Heresiarchs are spawned if you waste time instead of finishing him quickly. It's intentional.
Flechettes only bounce off shielded centaurs and invulnerable heresiarchs, the same way like in pure Hexen.
Death wyvern: that's intentional. It looks silly if you hide behind an obstacle, yes. But their movement changed, because they were very easy to kill with wings of wrath, just chase it. Now they're less climatic, but more deadly.
On the other hand, 11 wisdom/efficiency cleric uses no mana for serpent staff and can heal himself for free. But seriously, clerics flechettes are very deadly. The spirts within spell may be cheap, but it's a kind of weapon.