>>536091958
If the Sniper isn't cheating you can just pressure them and they won't be able to hit you and dodge your projectiles at the same time. Dodging in 2-dimensional space (by running around on a ramp) is also extremely effective and makes your virtually immune to headshots.

If the Sniper is cheating, you can harass them with splash damage while staying out of their FOV. Taunting after each kill makes them seethe.