I found Gate relatively easy. Just be careful not to fall, and your good. which is pretty easy to do, all things considered. Just dont be greedy and pick your shots, especially when he affects things like speed, or draws you to him. And try to stay a bit away from him later in the battle, since he starts destroying platforms.
Mijinion... Shouldnt be hard... but, those bubbles he makes are infinitely obnoxious. They take a bit to destroy, and draw your attention from Mijnion himself. And once you hit him with his weakness or a strong attack, he slams into a wall and replicates.
And his copies do the same thing, if I recall. Shoot bubbles and replicate when hit, as well as shooting that little tiny energy shot at you. His special weapon is the least of your worries. The Bajillion Mijinions are. And the bubbles.
Jesus hes a nightmare... Especially unarmored with no chips.