You should save the diamond cup for last. You get better as you play the game and it comes a lot more easily then. I think I got an extra or two after getting the diamond cup trophy without even trying too hard. Trying to go for it too soon is like banging your head against a wall, it just won't come.
The touch levels I still think are fun in single player, specially on the Vita (they're very boring on PS3/PS4/PC). That said, the game was clearly designed as a multiplayer experience for the Wii U, and that's where it excels. It's a shame we didn't get Vita integration on the PS4 version.
Overall though I find Origins a much better game. The checkpoint system is much more forgiving in Legends, and it negatively impacted the level design (they throw you into unfair situations all the time, since you can just instantly respawn).