I used Dr. Doom on every path though. There may be other options for some of the fights, but it's tough to beat his utility on the paths. For example, many say Sym Supreme is a better option as your mystic champ for the easy path. They might be right. But as you go through your other paths, you might be thankful if you can bring Doom. Korg can be a bit of a challenge without him, for example.