This part I have never struglled with. Dunno how people find it so difficult.
Anyhoo try to stick close to the wagon, like ride right along side it and never go ahead of it to clear some enemies. When enemies do come shoot, swing do whatever it takes to get rid of them but be do this whilst sticking close to the wagon either behind or on the side and only leave it to take out the boar riders but come straight back. Make sure to use the gale boomerang to put out the wagon if it catches on fire. There isn't really much of a trick to doing it you just gotta do it.
Hope that helps