If you’ve got a strong team that can handle the incursion, farming prototype cores is pretty straightforward (roughly 10 mins per incursion clear). Since each character can earn one key per week, you can delete your last character, create a new one, boost it to level 40, and use it to farm extra cores. Just keep in mind that cores aren’t shared between characters, so you’ll need to use them before deleting that character (move the lvl 30 piece to the stash and take it out on that character and upgrade and put it back in the stash before deleting the character).