What difficulty and what level? Without that it is not easy to help.
At least on Classic the Orivand fight is the second easiest of the boss fights. Kniles was more difficult in the EA but now the poor monsters in the caves need an eternity to get free, it's a cakewalk.
I sneak up the whole group near Orivand and the mages. Then start the fight and kill Orivand first (you may need two turns), then the mages. The female elf and the two melees need very long to attack you, so you have part-time superiority. It's not as boringly easy as in the EA because of the new you-AI-you alternating fight order, but it is easy.
If you play Tactician, it may be much harder but you choose it to suffer and suffer you shall.
