My favorite combo is Assassin/Sniper. Equip the Assassin with a bow (I use the Gastra Bow due to its high power, long range, and non-elemental damage), and you have a series of skills which work for all situations. You can use abilities like Beso Toxico (which is poison and full attack damage, versus the Fencer's Swarmsting, which only does half damage) and Doom Archer (if you're low on MP), to deal massive damage and effects from across the screen (Death Sickle is pretty nice too, especially against enemies with poor movement who won't be able to reach you before they die), then use Last Breath, Rockseal, Nightmare, etc. if the enemy closes in.