Rockstar,
(a.) I agree with much of Armen's advice, although some components of
Tactics are not for the feint-hearted!
(b.) I am not sure of your party's likely composition, although in
another thread you refer to a solo. As Armen suggests, going with something like a dual class with a Level 25 Assassin to a Fighter to get to the rogue HLAs will be best achieved in a very small party (as hideously annoying as she is, Aerie in a party of two might be a good option). As a solo, you'll be well served by setting up your quests so that you can pick up the quest conclusion XPs with your second class to get it to a managable Level 8-ish supplemented with some useful equipment.
(c.) My leaning would still be towards the multiclass Fighter/Thief, that gives you (easier) access to both HLA tables while your character levels up with no awkward dual class 'in between' stage.
Downsides, such as no poison weapon ability, no Kai, etc. I think are more than made up for with a more natural level development where you get synergistic benefits throughout the game. With particular regard to the Assassin-Fighter dual class, often the most difficult enemies are immune to backstabs and/or poison anyway. Personally I think a Berserker-Thief (cross over at Level 13) is a more compelling dual-class set up than a Kensai-Thief, as much of the early-game survival is the capacity to endure mind-based attacks.
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