Mitsuda is the longtime childhood friend of Yoshitaka Hirota (see below). And so, doing the friendly thing, he contributed a few tracks to the Shadow Hearts series. And those tracks happened to kick ass, nicely complementing Hirota's already-awesome work.Astaroth, for example, is among Mitsuda's contributions.
Mitsuda only composed one song for Xenoblade: the ending theme Beyond the Sky. All the other tracks were composed by the other composers including Shimomura.
Mitsuda worked on an obscure and somewhat mediocre title called Tsugunai: Atonement. You are not likely to have heard this song from that game, but you may have heard it from the playthrough vids of the Chrono Trigger fangame Crimson Echoes. It's a pretty kickass song either way.