Granblue Fantasy グラブル – Light Ultima Gun Team 光オメガシューター編成 Test Run − アフィリエイト動画まとめ
Light is another good element for an ultima gun composition because we have multiple powerful gun characters.
As you can see from the video it's hard to satisfy both Zooey and Robomi's need for buffs/healing (to charge their skills) with only Mary Christmas so I suggest you only bring one of the two in your party.
As alternatives, good picks are Maids and Silva. Both packs huge ougis and give other perks to the team (Maids for example bring gravity which can be useful for harder bosses).