Given how cheap the initial cost is and the big impact it has, leveling up this enchantment doesn't need to be a priority, but if you have a few creatures on the battlefield and want to let your hand build back up, you can focus on pumping mana into upgrades
This guy was basically adding 12 power to the board for five mana, so long as you had another creature to soulbond him with
Magic: The Gathering's Innistrad: Midnight Hunt set release date is just a few weeks away, so it's about time that publisher Wizards of the Coast kicked off its festival of card previews with the first batch of Innistrad: Midnight Hunt spoilers
My go-to combination is a +1/+1 counter and a first strike counter, with a heavy emphasis on obtaining shield counters from another source like Boon of Safety
But if we look further to Pioneer, I can see a lot of potential for this as the end game of the Twiddle Storm deck
The cost of Flashback gained through Past in Flames effect is equal to the cards mana cost