Well, the first option that comes to mind is having Imu Domi Reveri the Giant Kids so they can fight their parents directly. Honestly, it's a little strange that Imu hasn't already Reversi'd them, if it's so simple to do. It's almost like Imu's hesitating to do that for some reason. Either because that would suck away more of Imu's limited power, or because if they Reversi'd the kids they wouldn't be able to also use the Nightmares at the same time, or because kids may have some sort of natural resistance to getting Reversi'd for some reason, or because even Imu thinks using child soldiers would be a step too far. But, I don't know, that could still be a option.
Then again, maybe using the Nightmares a different way is a option. Instead of using the Giant Kids' Nightmares...maybe Imu has Killingham pull a Nightmare out of Imu's own head? Seeing one of Imu's Nightmares would be interesting. And I could see a immortal's Nightmare being somehow stronger than Nightmares that come from kids. This is probably HIGHLY unlikely. But, it'd be a interesting option.
I guess Shamrock could come back. But, I'm not sure what help Cerberus would be in this situation, if that's really all he has. A more interesting option would be for Garling to show up, with his new "Yokai" form. But, honestly, even that doesn't really seem like it'd really turn the tide here. You'd have more fighting power. But, they need leverage on the Giants, not strength. Adding more soldiers doesn't help convince the Giants to join your side. And there are better options to use, if you want to just kill them all to take them off the board.
Another option would be to just destroy Elbaph altogether. If you can't recruit the Giants to your cause, you eliminate them from joining up with anyone else in the future. They could just use "Uranos", like how they eliminated Lulusia. But, they only JUST got the Mother Flame from Egghead, so maybe York hasn't had time to install it in the Ancient Weapon. I suppose another Buster Call is always on the table. But, it would take time for the Marines to get to Elbaph.
So, if those two options are off the table, there is a third, crazier option that comes to mind. Could Imu just...throw Egghead at Elbaph? Could Imu use a Abyss Mark summoning circle to throw a entire island at another island? I mean, if Imu already has the Mother Flame, do they care about anything else that would be on Egghead? I don't know. I just imagine this gigantic summoning circle forming above Elbaph. Then we see Egghead come through from above. And then Egghead just DROPS on the Elbaph destroying everything. Again, this is HIGHLY unlikely, but it would make for a really fun visual. And Egghead coming back as a "weapon" would be a interesting way to escalate things.
Could we see a hint of Imu's "Yokai" form via Gunko? I could see Imu trying to stick around while the others retreat. And if Imu's "Yokai" is some sort of fire-breathing dragon, that is a pretty good threat to Elbaph, which is famously prone to catching fire. Imu trying to burn down Elbaph while everyone else tries to stop them.
Or, less dramatic but still making things interesting, maybe the God's Knights just steal the Thousand Sunny to take the Giant Kids away in? I mean, the Sunny IS a part of the crew, and the God's Knights need a boat. And kidnapping one of the Strawhats is usually how you raise the stakes in a arc. The problem with this is...I don't think you can fit all the Giant Kids on the Sunny. The Giant Kids are just too big. But, you could just kidnap...maybe 3 of them? I like the idea, but I think it has some practical problems.
This is also pretty unlikely. But, I haven't totally given on on my "Imu is Mosa" theory. Yes, "Mosa" is probably Shirahoshi. At this point, it'd be a pretty weird red herring if she wasn't. But, if Imu WAS Mosa, that would give Imu one last chance to appeal to Loki to join the World Government. And it would give us a nice bit of dramatic tension to see who Loki sides with, his only friend or his country. But, I'll admit, most of that tension is gone with Loki's good name being cleared by Jarul. So...yeah, not likely to happen.
But, I think the most likely option is just to retreat. This whole mission has gone FUBAR. Mariejois is on fire. Loki and Luffy are coming, eventually. The God's Knights probably just need to cut their losses and run. Before that, I do think they'll set off some sort of a bomb or something that the Strawhats have to deal with, to prevent the destruction of Elbaph. But, I think most of them are just going to end up running away.