I've been going through the main campaign for a while now and have a level ten character at Chapter 2. Originally I had planned to go through both expansions with him as well, but I just found out that SoU is designed for characters at first level. I know I can just go in with a higher level character anyway, but would that ruin the difficulty? I can always start a new character for the two expansions if need be.

It certainly would ruin any challenge in SoU if you use your 18th level PC from the first campaign.

However, you can certainly use that high level PC in the HotU expansion campaign, as that one is for high level PCs.

I could use a chance to make a new character anyway, so this should be good for me. Thanks for the quick response!

Arcane Archer prestige class was introduced to NWN in SoU, looks like the levels were almost built for the class as well. Loads of fine archery opportunities.

A fun class to try out in any case, requires a bit of forethought to meet the requirements.

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Also might not be the best possible class to do HotU with, but then you could bring in the original character from main campaign instead. Or make a new one there as well.

I recently finished a playthrough of HotU with my Arcane Archer (9 Monk / 1 Wiz / 29 AA / 1 Monk) pretty handily, even with Tony K's AI mod and Hardcore difficulty. AA is a fun class, though you may have to do a bit more scrounging for items so that you can afford ammunition until an unlimited ammunition weapon comes along.

...afford ammunition? The point of Arcane Archer is you use plain arrows!
1 GP per bundle. At AA level 1 those act as +1 arrows and +5 at level 9.

Twang! Twang! Twang! Pointy death!
:D

If I'd play SoU again, I'd probably do a classic Elf Ranger/Wizard/Arcane Archer build.
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Heck! I think I'll do just that...

My sorceress acompanied by deekin makes pretty funny gameplay through SotU and HotU now. I think I have found bow with unlimited magical arrows you guys talking about :-)...

I started the OC with a straight paladin and wanted to see if I can solo it. It's making me wonder if I am missing some good lot because there are some chests that can't be bashed open ("weapon ineffective"). Can I bash them open somehow? Will I regret not having Tomi or some rogue skills?

You probably are. But nothing you'd miss in later stages.
Better to find an "open locks" magic item from a magic shops rather sooner than later.

And... you're just not getting through those chests/doors damage resistance. Get a bigger weapon, learn power attack, get a weapon that does elemental damage. I usually lugged around a greataxe or something like that. A big hunking thing with a big critical hit multiplier, so the criticals can break stuff normal damage can't.

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Even +1 point of fire damage usually does the trick.

Yep, in almost all cases that you get "weapon ineffective" it just means that you need to do a bit more damage to overcome the damage resistance (or get a weapon with a higher level of enchantment). Basically just keep in mind the wisdom of Minsc: "No effect? I need bigger sword."