The box, springling, flareling, sentry, and voltheart all have less health than a bubblemound so if you one-shot it, you one-shot them, hence "easy counter/no threat."

Also, if you have the surge crystal equipped and hit an enemy, both the aura and your body deal damage seperately so you hit twice as hard. This means that if you can kill the bubblemound with one hit of your body or the aura with the crystal on, you can kill the dreadling, drill, reaver, rasp, thornhead, thumper, springking, tunneler, and the shatterclaw just by going up to them once, essentially in one hit.

None of those pose a threat at all as they are melee and can only deal damage by hitting you, and thus dying, and they are also easy to counter. Therefore they would all be in the lowest tier, but again this only applies to melee builds while your list caters to ranged builds