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Issues:
1. My damage and hit-chances with melee are absolutely crap.
This is something I've never come across - I suspect it is because I min/max str for melee chars (never less than 10). In fact, even using the melee stances that provide -% to hit chance, I rarely miss even on boss mobs.
It might be interesting to list your stats for us.
2. Combat targeting is stupidly fiddly.
This seems to have got worse in the last build ~ a lot of people have commented on it, including me.
3. I suspect I chose poorly when I created my characters- enchanter and assassin seem to be vastly underpowered.
You've chosen the two 'worst' builds, for sure. When you say "enchanter", what do you mean? Crafting build?
4. Also on the obnoxious side is that clicking on a character who died in the poison from the zombies intending to rez the character gathers the entire party around them
Lots of people have complained about the new formation auto-feature. I suspect it'll be tweaked, or at the very least be optional.
In short, combat in the game so far is a mass of frustration.
The early fights (west of town, zombie groups, not north or east!) have several things that slant them in the player's favor: notably barrels / exploding pumpkins and so on. Scrolls, as far as memory serves, do not require skill checks to use - try investing in a couple of summon scrolls. The NPC mage, Jahar, is very weak early game (better used as a mini-healer), so without a dedicated ranged DPS char you're a bit stuck.
You can also (reportedly) abuse the stealth mechanics / smoke bomb teleport as well, but I've not played a rogue.
At the moment, you're correct on each of these points; the game really 'opens up' once you start using summons and is fairly harsh towards melee stealth atm. However, you mentioned that you have both NPC party chars - I'm surprised you're not using rush / bull + healing to get M as a tank into the fray, and your melee stealth char to backstab.
This is rather suggesting I should do a play through going for "
worst possible builds" and see how it goes. Maybe next patch.