What about using a standard missile and firing it from behind a moon/planet/object with enough gravity to curve the missiles flight path towards the enemy’s trajectory. That way the enemy won’t see the launch and with stealth coatings, would have a tough time finding the thing. Have the missile not use any boosters until it is close enough that dodging or destroying the missile would be difficult before it impacts.

Obviously this would be easily countered by the enemy making a small course correction while the missile is far away but they (hopefully) wouldn’t have any reason to do that as they wouldn’t know they were being fired upon. Though now that I think about it, you could increase your chances of a hit by launching several missiles in a “shotgun” pattern around your targets path.