As I recall, this was a tough fight for me. And I had a full party including Minsc, Jahiera, Yoshimo (the thief you mentioned), Airie, and Nalia. When I faced them I had already finished a handful of quests and had leveled up a few times. At this point it was still a tough fight for me.
It sounds to me as if you are very early on in the game. My suggestion would be to get some experience and come back later. They're not going anywhere. But if you do decide to continue trying....
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Hit that stinking mage with spells like "insect plague" or perhaps "silence 15'" before he can do any damage. If you can take out their mage early on, it will be a much easier battle.
Good luck!
