If that’s not bad enough, one could envision flinging appropriate small moons or large asteroids from other solar systems. Why not? Boosting a large body to fractional c velocities and aiming it at a far away planet isn’t inherently any more absurd than building a ship and boosting it to fractional c. The motion of stellar bodies is famously predictable out for many centuries. The body could guide itself: add some small motors, and it could even correct for small errors in prediction. It would be the ultimate fire-and-forget weapon of mass destruction.
So you don't have to sneak up at all. You just need to have a solar system with a Kuiper Belt, and you've got all the arsenal you need. In fact, launch attacks from remote solar systems is preferable, since it gives your bullets more time accelerate before becoming visible to the enemy. They may have enough enough energy to stop an asteroid traveling at .5c. Do they have enough energy to stop one traveling at .9c?