most people find either a grey dwarf or skeleton spawner. I like the draghr villages, where you can get 6+ spawners at once, or those piles in the swamp, where there are 3 or 4 in one place at the weird rock formations.
Find those and 'aggro' the spawners, and typically I build a couple of wall pieces to hide behind (this makes the archers come to you and helps prevent them from shooting their own spawner). Then you just sit there and kill for a few days. I suggest blocking each enemy at least once, maybe twice, before killing it.
Its possible to train block off ballistae; set up 3 or 4 to shoot at yourself. Its a lot of wasted ammo, though. For this you want to put a block behind you (raise terrain or stone block?) so you can ignore the knockback effect.