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This battle is set before the Glitchtale events. This battle involved Sans and Flowey.
Flowey seems have to finished his genocide Route. Flowey then absorbed the determination orbs in The True Lab and turned to his Omega Flowey form for a while with limited power. Flowey's last targets were Sans and Asgore.
Sans was standing in front of Flowey, in order to stop him and protect the timeline from Flowey.
Despite the fact that Flowey possessed the ability to RESET and Continue. Flowey's abilities are limited and Flowey cannot use most of Omega Flowey's powers and Abilities, So Sans overpowered him due to his capability to bend time and space, karmatic retribution (Flowey had 9999LV which meant the karma would effect him.) and also that Gaster blasters are way stronger than the quantity of weapons that Flowey showed.
Also, Flowey was not in full Omega Flowey form and he had limited power. When Flowey gave up on fighting Sans and went back to his normal form, Sans picked him up and played a game of "Spare, Don't Spare." (Similar to the game of "He/She loves me, He/She loves me not.") Sans started plucking out Flowey's petals one by one until he ended up with don't spare, then to this, Sans killed Flowey again.