With SWORD invading Blackbeard’s territory to rescue Koby. I believe it will end with Garp’s Unexpected Way of Defeat and his Glorious Death similar to Roger and Whitebeard.
Reverse Marineford style
Garp is shown to be one of the strongest Haki masters so far in the story and against a crew of fighters relying on their devil fruit. It’s a bad matchup so brute force will simply not cut it.
However, Blackbeard and his crew are known for using cunning and underhanded tactics against opponents as well as their devilish luck. The key here being Perona and Gecko Moriah’s love for his crew. The way Garp gets defeated would be through Gecko Moriah stealing Garp’s shadow at the end of the fight when he least expects it.
Of course, Gecko Moriah wouldn’t help Blackbeard willingly for killing Absalom, but Shiliew and Avalo Pizarro can easily make this happen. Avalo and Shiliew can track down Perona and Moriah while they try to escape, subdue them and take Perona hostage. Forcing Moriah to cooperate with the Blackbeard Pirates.
As we can see how much he cares for Absalom, Moriah wouldn’t let Perona die too. Garp would disintegrate if his shadow gets stolen seeing how the fight is currently about to take place during daytime.
Of course, strong Haki could negate fruit powers as we saw in Wano as well as Law vs Doc Q. But it is also a finite resource as we see with Luffy’s Gear 4 Haki can be drained and needs to be replenished.
Garp fighting an entire emperor crew with thousand of pirates in Beehive while protecting his subordinates would take every bit of Haki he has.
Just like Whitebeard Pirates in Marineford, they would need to escape at some point due to the sheer disadvantage. So I think at the end just like the twist with Whitebeard dying before killing Teach, Garp would get his shadow stolen at the end of the fight as a twist right before he wins almost out of Haki.
He keeps fighting before disintegrating to ensure SWORD escapes as his final heroic act. As carriers of the will of D. don’t die fearing death.
Koby and the rest of SWORD escape as Garp’s will is passed to Koby, the new hero of the marines. Atoning for not being able to save Ace, he saves the good future of the marines in Koby for the world like a true hero to the end.
Just as Whitebeard Pirates ensured Luffy escaped to ensure Roger’s will lives on through Luffy the good future of the world. SWORD gets a successful rescue at a heavy cost. Garp dies gloriously ensuring a positive future for the marines, just like his friend and rival Gol D. Roger did for the pirate world.
*Theory by ZestycloseCake165