from this review http://gbatemp.net/review/abyss-odyssey.162/
However, co-op was an entirely different story and I had a hard time enjoying it, especially when playing online, which had more than its fair share of performance and gameplay issues. Over local and online co-op, the biggest complaint TankErdin and I had is that you are fighting together to reach the bottom of the Abyss, but you also have the ability to attack one another, which further complicates gameplay and makes it absolutely chaotic, and not in a good way. With stronger enemies or bosses that now have an extra enemy tagging along, one wrong grab or poor swing with an ice or poison weapon can easily turn the tides against you for something virtually unavoidable. The only way to really prevent this is to have each player draw an enemy to their side of the screen, which doesn't always work, or to have one player sit out at the side of the screen while the other takes all of the heat, which isn't fair to either player. On top of this, player two has no incentive to join a host other than getting achievements, adding to their Warlock kill count if they get there, and otherwise just screwing around. In our playtime, TankErdin and I found that the person joining an online game neither gains or loses any experience, levels, or gold after they quit. And beyond this, online play suffered from many synchronization and performance issues, with characters stuttering across the map, being stuck in a jumping animation and being able to fall through the floor, and the camera not being able to decide which player to focus on and letting other players run entirely off screen, when ideally it would follow each player separately. Playing local co-op solved all of the synchronization issues, but the camera issue still remains.
I'm gonna skip this one.