Fifty-degree Banshee fire
To perform the glitch, play any level with a Banshee (i.e. Assault on the Control Room or Two Betrayals). Fly the Banshee to a tree and fire its main weapon while facing said tree and while in close proximity to the tree. All your plasma will barely graze the tree and shoot upwards, so you could, in theory, take out units directly above or below you.
Although this is possible to do with the Banshee's Fuel Rod shot, it is not safe because the Fuel Rod has its own set of in-game physics and thus is affected by gravity. As such, it may sometimes hit the tree and damage the player.