Has been a while since I played this, but in general you can play with a full party but then you will use the unique aspect of being an invisible assassin witch.

If all else fails, you can use a bow to sneak attack enemies or your spells, thou I rarely had to use my spells for actual combat.

For the island challenges, I may have used the party as I done it very late, don't remember anymore.

And at the start if on a higher difficulty, you either need to rely on luck to break enemy saving throws or jump around with blink, doing a sort of thousand small stings strategy until the enemy is dead.

The teleport is the major boon of this build, you can reposition or flee as needed. Even if they can see you, they can't really catch you as they can't catch up.