Either straight co-op or straight solo. You can't mix and match, unfortunately.
I am guessing co-op is alot easier though right?
I found solo easier. Even though you restart the level if you die. It's easier to keep up with just your own actions instead of someone else's too. You could also do a really fast "save & quit" if you die and keep trying it that way. I imagine it would take forever with that method. --- GT: Incredigasmic Finished Pokemon Black National Dex @ 85 Hours and 15 minutes on 04/08/11!
It depends on how good you are and how good the people you try and run it co-op with.
LASO solo sets you back to the beginning of a level if you die LASO co-op sends you back to the last checkpoint if anyone at all dies.
So if you get some buffoon that rushes into the mobs and dies over and over you'll never make it anywhere. --- By the edge of those wilds, warriors have charged. But now the years have passed, the generals have left and the grass grows in the armors hollow on the hill.