Fire Kengo with Yukata Silva, Schrodinger grid (Sandbox FA) / [グラブル] 火剣豪 シルヴァ ガルーダミーレス 

Schrodinger is a surprisingly good off element grid weapon thanks to its Blade Wrath skill, which allows for a Katana centric build that is F2P (no need for multiple Ixabas) and enables very well Shion for her passive bonus.
The addition of Yukata Silva makes Shion extremely powerful, as she functions as her battery, and the different damage multiplicative debuffs (Silva's ougi, Fatal Chain, Fraux's debuff) are on top of a grid that caps as much as possible all the various CA enhancements (supplemental, CA cap, damage cap).
With the guidebook, it makes it plenty doable for characters to reach the 13m boosted hard cap, and even characters without an Unworldly modifier ougi get very close to it. I suddenly kinda wished I made Alanaan NWF since it fits perfectly in that comp...