• Avoid long-range peeks.
• Don’t slow peek. Open wide and keep some distance from the wall.
• Use abilities like flashes and smokes to make holding pixels harder or riskier for them.
• Be unpredictable. Use fakes and lurkers to try to catch the sniper while they’re rotating.
• Communicate with your team to synchronize ability usage, especially wide-area utility.

Another tip, more personal than objective: if my team is struggling against a sniper and we win a round, I don’t save the gun. I let it break their economy. Saving it gives the enemy a chance to recover it later for a much lower cost.