If you don't have someone to play local co-op with you, this can be done with two controllers if done carefully. Obviously, never let a player die if their lives are at 0, otherwise you'll have to replay that level. Both players must start and finish every stage.
I recommend the Balance and Spin Kick Tapes maxed out. Don't bother with dragon swarm as it causes the game to freeze, sadly. Also, as a backup for level 9 if you struggle max out Training Wheels because the undead enemies seem to do much more damage.
What I did, controlling two controllers at once, was spin kick at the same time then gleam, rinse and repeat. Even on the highest difficulty, a gleamed-up spin kick will make things much easier, and bosses will be no problem whatsoever. At shops buy as many lives as you can so player 1 can take some hits, and keep player 2 alive. Things will be pretty easy until the later stages, which may take a couple of tries. Fuzz face helps a lot, so abuse that character when you can.
Note: spamming the gleam move can freeze the game during the boss fight at the end of stage 8 (plant-like Marian II) . To avoid this, take out the left or right side first completely before you engage the other side.