>>18160419This is the big one, except it doesn't take more advanced technology than what we already have. Once you have a platform in space that can use gravitational assists, you can get up to a speed far below c but which carries the force required to cause tsunamis tall enough to wipe the face of a planet clean, to put enough dust into the atmosphere that all plant life will die. Life might survive, but humanity and most advanced organisms would not.
The only way humans could possible counter aliens is by getting into space and gearing up ourselves to shoot our own ultra fast bullets. The dumb liberals who say we should pay for niggers on Earth instead of arming our species should be skinned by Mexican cartel members for threatening the lifes of everyone on Earth.
You could speed it up to the point where it was relativistic and no one would see it coming, but there's really no need. Once something is going fast enough, it doesn't matter if you blow it up into pieces - now you just have a bunch of shrapnel going at the same speed. It's also unrealistic to suppose any lifeform in the universe is ever going to have the resources to vaporize an asteroid, so these shrapnel pieces are still going to be huge.