If you do it with lvl 0 should be easier as the AI will be the same level.
You can do it co-op yes, however I am not sure this counts to your trophies. I think when I had it, I had to do them on my own to get the trophy.
Hopefully someone else can give you more insight.
Not a bad game for a short timeframe. Not a keeper though.
Doing it in co op will not give you the trophy. As said before, do it with a new character, it will make it alot easier.
^ This and that +1.
New character, no co-op.
Try a heavy, mobility is not a problem...