The backyard achievements were my most favorite in the game. There's only 4 enemies per class and the achievements aren't for killing them over and over - it's for killing them a certain way. It makes it both fun and challenging. I highly suggest you going to the backyard!
One problem with the games achievements is EVERYTHING is linked to the Temple of Enlightment, which is a long, and tough dungeon (unique though). By beating it, you'll get the last spell (needed for the Spell Achievement), a few treasures (needed for the Treasure Achievement) and in the dungeon is the item to unlock the Super Heavy class in the Backyard (needed for both the Super Heavy and Immortal class achievements). If that doesn't make things worse, you'll need to get the backyard achievements and collect all 99 seeds to get the last 2 accessories that have skills that you'll need to learn to unlock all 4 skill achievements.
So to wrap things up, you'll need to beat the Temple of Enlightment in order to unlock 8 achievements. :(
But seriously, the Backyard is far from a snooze fest - it's short and well-thought out - give it a shot. Oh, and you do get about 12 skill nuts, and a kick-butt sword and ring for Kaim by beating Immortal Class in the backyard, as well as an item that absorbs ALL elements - wahoo! I only wish they let you try all classes in the backyard without having to 'unlock' them.