If you're a human you can dual to fighter but you'll gain no more exp. to your theif class!
Otherwise you'll have to create a new character (unless a bit of shadowkeepering is done
)
I like to position my theif behind the mages of the party(s) and backstab them, which will hopefully take them out whil the rest of my party moves in to mop up the rest. But thats me.
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[insert witty phrase here]