Lets talk heights for a second here, Normal canon sans is like 4ft tall so im going with 5' for this and I made underfell about 5,2 and I made G.Sans about 6' tall so he is quite tall. 

Sans and Underfell where just chilling watching tv "the MTT program" when Underfell climbed out of the trash, "Sans, G has nothing, especially if we both fight him at once Y/N would totally swoon to us if we kick out the giant" Underfell cracked his knuckles with a confident look "Yeah, hes basically us so its 2 against one... lets go" Sans turned off the tv and got up with a stretch. Underfell put on his sneaker and cracked his neck prepared. The two  went out back to find G smoking something. "whatcha' smokin?" Underfells red eye flickered. "incense, cigarettes are bad for your entire body even your bones so thats why I do alternatives" He blows a little silhouette of smoke. "What do my little me's need?" He teased.

"Lets fight" Sans blue eye flickered, "fight?" G's golden eye flickered curiously, "Yeah! Me and Sans fight you" Underfell growled a toothy grin. G laughed "oh?" He laughed. He blew one last smoke cloud then pinched the dust snuffing out the flame. "Alright, lets go somewhere less disruptive to the people of the town" G began to walk off. Underfell and Sans looked at each other and followed there taller twin. G stops in a open snow area "Its a snowy day outside..." He says quietly. "birds are sleeping, Flowers are resting... on days like this... You kids will have a bad time" G Sans spun around as a wave of black bones rose out the ground, Underfell jumped into the air and summoned a sharp bone blade and sent it back to G.Sans. 

Sans lept out of the way and summoned a Gaster blaster and sent it to attack G but suddenly its obedience changed to G after a stare and it came back attacking Sans. G smirked and summoned a a coal black tentacle magic and used it sending it at underfell then he pointed it and did a simple hand motion and it split into 4. Underfell growled using his sharp bone blades cutting back these weird things back. Sans lunged at G but was stopped by black hand magic. Sans freaked out at there pinned him down "UNDERFELL AIM FOR HIS HAND, HE HAS TO HOLD IT TO KEEP ME PINNED~" Sans yelled. 

Underfell smirked and aimed but was suddenly caught off guard by G's magic, "Come on you two... I thought I taught you better" Gaster smirked as the two struggled to escape his grasp. G chuckled softly and summoned little tentacle things holding feathers "now your gonna have a bad time" He smirked. "WAIT NO STOP!" Underfell began to freak out as they began to tickle them. Sans tried to keep composure but just started laughing and could barely breath. Underfell was making silly faces as he bit his lip until he started giggling cutely. "better... better" G laughed. G was suddenly suprised when he notcied he couldnt move his feet. "boys... come on..." G sighed as he felt the things wrap around his legs "come on you guys.. i-it was a joke come on" G was chuckling slightly. He began to loose grip on his magic "c-come on you guys~!" G collapsed in a fit of laughter. They let him go after a moment

"Alright fine, you guys are good fighters" G smiled softly as the three boys returned home.