Aegis Defenders is probably one of the tougher games I've played in recent memory, and that's all thanks to the tower defense sections.  Fans of that genre probably won't have too many issues, but newcomers playing alone might find those sections pretty daunting until they master switching characters, learning the arena layouts, and know what traps to use when.  If you have a friend to play co-op, you'll likely have a much easier time.  Still, it's a pretty good game for GUTS Department's first outing.