Aneska has a card where she summons the mech when she's wounded. With a five stack she -always- summons it at full health. So you can strategically wound yourself to keep the mech around.
Also. I feel like you want the mech to be a fifth character... You don't need a fifth character to play this game. Consider that this character has pretty great clear on all of her weapons, and can destroy swarms with just her weapon.
How much more can her mech do and her not be doing too much?
How your describing Aneska kinda reminds me of Iune, because to me it sounds like the mech is like a second ability.
Also, I wouldn't say all of Aneska's weapons are great, but most of them are good for swarms and with each weapon they either fire projectiles or have a mech ability that is good for attacking over corners; personally I prefer Dr.Kuzman's Lightbogen ZXX for swarms, because damage amplifies with each member of the swarm, you can hide behind cover while the swarm continuously takes damage and when the swarm electrocutes itself, your stress meter doesn't activate. Furthermore, unlike most of Aneska's weapons, the Lightbogen ZXX doesn't damage him. I think it's range is between the Aneska's Atlas G-620 and her Ionizer K-23, though the ray can go farther if you equip a certain talent.
I don't think it's fair to compare the blitzfaust to the mech ability, because it changes based on the weapon she equips.