Get toughness & agility to 5,the others don't matter, unlock the +30% defence thunder hammer, & do the 1st mission of another campaign, with the AP you'll have its a sprint to the finish you'll breeze it as your over levelled. if you prefer a more honest approach psy barriers are the way to go high agility still a must. That's for the achievement melee for normal play i found not viable as they get left behind & die.

Use the space wolves chapter. They are a better chapter to use for melee, HOWEVER, you do need to level them up a fair bit. So stick to SBs for the time being, until you you get toughness and agility to their highest.

Then go axes. Still, SB is still generally better, as you progress, you face tougher enemies. And acid ones, But there are counters to that one.