The "Monster Purge" quest for Character Slot Coupon is bugged.
If you change channel (including entering or leaving cash shop, monster life, evo, dimensional invasion, cross world pq, or any other "private channel"), or disconnect/logout after accepting it, the quest will disappear and you will not be able to continue, forfeit, or re-accept it on any other character on your account.
So, only accept it when you are already on the map where you plan to kill the 300 mobs, and do it as quickly as you can.
Edit: there appear to be reports of Cassandra's Giftravaganza ("hold this marble for an hour") quest interfering with this one. So don't try to do them both at the same time.