Fickle and cautious, the Muktians are unique among the alien races of the Purple Void in that they can be friendly as well as hostile. You probably shouldn't attack them if you can avoid it. Making friends with a Muktian pilot or captain and bringing him back to his fleet is usually preferable to starting a galactic war.
In combat, the Muktians can be a formidable enemy although their fighter craft are somewhat weak and their subterranean heritage doesn't make for natural space pilots. The backbone of the Muktian fleet is the corvette, armed with long-range nuclear missiles that pack a deadly punch.
If you really need to attack these guys, here's how: if you have allied fighters, send them out to draw missile fire. Although devastating on impact, the heavy missiles are slow and usually can't hit a fast moving target. This will give you time to first deal with the incoming Muktian fighters and then get up close and attack the corvette. Attack from the side if possible, so it can only fire one of its weapons at you. Use your fighters to attack the corvette from behind.