I'm on my phone right now so I can't really post a deck list. But what you want for a budget swarm deck is zyx, gloomchaser, wraithling swarm, one other 2 drop (I suggest bloodtear alchemist), then bloodtide pristess and shadowdancer; you can run shadow watcher too if you want . For spells stuff like soul shatter pact, deamonic lure and ritual vanishing. Toss some dispel in there like light bender and you should be good to go.
When you get enough spirit I highly recommend crafting deathfire crescendo and after that spectral revenants.
The main mistake I see when other people play swarm is that they take the mans tiles when playing as player 2. What you want to do is summon your minions behind you so you can take advantage of their deaths when the enemy general moves forward.