Our honest take: buy singles for a deck's core cards where you need specific pieces to function
Illustrated by Tom Babbey Standard Legal Alchemy Not Legal Pioneer Legal Historic Legal Modern Legal Brawl Legal Legacy Legal Comp
Second, apart from tribal Cats (which may or may not be possible, due to the lack of cards), the only other direction to go is equipment matters, which tends to be an expensive upgrade, since you'll want various Swords, Batterskull, and Stoneforge Mystic and friends
Sazhs Chocobo Im starting this list with a cheap card that also happens to be adorable, and thats Sazhs Chocobo
Gwen Stacy/Ghost-Spider (Borderless) Sticking with Gwen, but a much less spooky version, this borderless card shows our heroine relaxing before flipping into action against Electro
For example: You'll notice summons from FINAL FANTASY XIV usually have the word "Primal" in their name, as in the game world of Eorzea, many denizens refer to summons as such