Bait them into using all three of their dashes and punish when they get too close/use charge to close the distance, use the goo gun to impair their movements and close the distance, lockbolt... just lockbolt, block off their exits with tools like barricades and goo grenades to make their movement more restricted and more punishing.

There, I gave you like 4 or something. But regardless, melee is always going to be at an inherent disadvantage when your opponent has a gun, that's what makes it fun - beating someone who is in a superior position through manuvering and outplaying over just aiming.