I prefer Harec's base gun as a Raider. It has good charging power, and with solid aim can clear mobs fairly quickly. With his blue gun, you don't have to fully charge it to get kills, it's simply that powerful. Headshots are basically instakills, so go for that. Using his ability is all about line of sight, you want to figure out which spots are the safest and provide the most view of the battlefield. This is really easy on the Prologue as there are towers and high spaces that are hard to reach. He is also super effective against Kuzmann, just jump from pillar to pillar.
Avoid melee as much as you can, only rely on suprise in this regard. Like if an Antag moves near your return point while you are Stalking, return to it for a stealth grapple. My favorite card of his is the one where you get immunity after every kill while stalking. It helps you bide time against large mobs that discover you or even antags. Just stay calm and keep killing mobs.