The Gray Stalker is a hard enemy and the first type of Stalker Aya meets. The Combat Lights are useful because it causes it to not only drop from the ceiling but also become visible, allowing Aya to attack it easier. The Grenade Pistol is very useful against it because it will deal 200+ damage without Energyshot and can stun it partially, allowing Aya to get in more shots. It can inflict CONFUSION should it throw Aya at the ceiling, so Metabolism is useful.
It is possible to inflict PARALYSIS on the stalker for 15 second with Pepper Spray (less with PE spells), but it requires that the stalker is on the ground before that can happen.