From Exterminatus Manual
Tyranid Genestealer (Stealer)
Genestealers are the infiltrators and shock troops of the hive fleet. Moving years or even generations ahead of the hive fleet, the infiltrate and corrupt the civilisations they find, weakening them from within. Genestealers are the smallest tyranid creatures on the battlefields of Exterminatus, but they lighting speed, amazing agility and rending talons more than compensate for their small size.
Genestealers are armed only with their Talons, leap ability and developed cunning. Primary attack lashes out with the talons, make many rapid strikes. genestealers can also leap by pressing the SPRINT an JUMP keys at the same time. Leaping makes previously inacessible ambush spots reachable. Leaping drains all the stealer's energy, preventing them from making sucessive jumps, but they can recover quickly.
Stealers are one of the only class in Exterminatus that doesn't have a gun, so you'll have to rely on your small size and high speed to get in close and use your talons. Use your leap to get behind the enemy lines and ambush them, it takes very few talon strikes to take down a marine.