You're going to find marauders amongst the easier kills as a Sniper. You have to be really proactive about popping your defenses. Don't wait until you're at 10% hp to pop Dodge etc.
Slightly off topic: Sure dying can suck but sometimes it's the best thing you can do to help your team (ie: stopping a cap even if it means dying). Frag Grenade and Flashbang are awesome for breaking up zergs.
The real problem for 1v1 lies with Shadows/Assassins and Operatives/Scoundrels as they can stealth right up to you. Against the stealth you have next to zero time to react before you're flat on your face getting hammered by the class.
Happy Sniping,
-Calitrix