It's a long game, so it'll be a bit before you fill out your full party of four. Just have one in the bag and use it as a basic attack. I wouldn't spend points on the boomerang tree, personally. I tend to have Hero go mixed Sword/Spear, as swords give you some great utility options (Falcon Slash and Miracle Sword in particular) while spears let you get in on the slime hunting as well. So it's not a bad thing for him to specialize in.
Axes are also where Yangus can become a metal slime hunter via Hatchet Man and Executioner. You're going to be using Helm Splitter (Yangus' early axe attack) a lot throughout the game to consistently drop enemy defense. There are a few basic skills that can really help in the long run.
