What is melee? I was playing and my opponent said he would melee and make a close combat attack up to my higher elevation. I didn't think this was legal.

Melee is not a term used in heroclix. Close Combat action/attack is probably what he was going for.

If he could use Giant-Size or Leap/Climb, OR you were on the top of some stairs/ladder/what have you and he was on the bottom of the stairs/ladder/what have you what he did was legal.

Otherwise, it's illegal.