You make the spearton hold, The miners goes at the back of him and hits him until he kills him. same to swordrath, spearton, merics. Shadowrath can kill spearton cause the level is higher. Same goes to Everyother unit.
Just because they're higher level, doesn't mean they instantly win. And there is no such thing as a unit's level in an RTS. Each unit generally serves a different purpose. It's not like one guy has a stick and another guy has a gun.
And I'm not sure what you mean when miners go at the back of him and hit him until he kills him? What?
Anyway Speartons are a fairly useful unit. They lack a hard counter unless your opponent just masses them. If they do, they albowtrosses with blazing bolts and a magikill can work if well used. Electric Wall on self, poison then explosion.
Otherwise, swordwrath are much more effective damage wise. With rage, swords are much superior to speartons, due to lower cost, faster speed (with rage), higher damage (with rage), Less Population use, Faster Build time and the fact that they are more numerous, which means if one dies, it's not as big of a hit. Speartons are only better than swords because they have higher health (makes poison less effective), have armour (reduces projectile damage) and have a slightly faster attack rate (if swords don't use rage)
But yea, rage swords and/or albows (blazing bolts) with a possible magikill is basically the 3 weaknesses of speartons.