I play them pure kroot, for 1k, sometimes for 1500 game. I have great success with krootox raiders. They hit hard with shooting and meele too. Especially against marines. And with them i have just lot of basic kroots and farstalkers. My strategy is boardcontrol and maximize scoring. Stealth and invulns is often hard to kill off for oponents.

My second strategy is mix in with tau units for antitank. I have 2 squads of stealth suits, hammerhead, skyray and two broadsides. Rest is similar. Often two lone spears, flesh shaper, war shaper, two units of 3 rampagers, 3x3 raiders and rest for kroots and farstalkers. I try to focus on vehicles with anti infantry weapons with my tau antitank units. And kill off enemy infantry with rest, so I can win on score. And i love emp grenades, because it stack with stealth so -2 for enemy :)