The main thing that makes Skorne difficult to defeat is his ability to shoot seeking skull projectiles. When a skull is flying toward you, do a quick, tight loop. The skull will continue for awhile in a straight line. By the time it turns around and gets back to you, it may have disappeared. Other than Skorne's skulls, he has no attacks. As soon as you 'trick' a skull, start blasting at him with your long range attack, or use potions, especially if you are a wizard.
Garm is similar to Skorne, with only one attack (fireballs), but has the ability to unleash several at a time. Use the same strategy as with Skorne, but it may take more time as you have to dodge more projectiles. Do not try to finish this battle quickly or you will get pounded by fireballs.