He first do Blue soul first, until he switch into Red, the fight keeps on with the same result.
Due to his Karma Retribution, and Poison, it makes the Fight harder. Being hit by his bones, or Gaster blasters only reduce you 1 health, but with poison, it reduce more than 1, depends you stayed.
His fight is very similar to Sans' fight, except he reduce more hp, his a bit hard, He also wears Papyrus' sweater but it was wrapped in his neck. To progress the Fight, using the FIGHT button is the only way to progress the fight, using other options than Fight didn't progress the Fight. He somehow talks during the battle. However, he offers mercy.
Accepting his mercy might do a betrayal. Like other users/players doing the betrayal route, it means you offer them mercy at first. But you instantly kill them when your almost done. Pasting his mercy offer progress the fight normally, but at first, the screen flickers.
He always talk about his 'Special attack' in the halfway of the battle.
He disabled INV with Karmic Retribution, instead of staying like other enemies, he started to Dodge. His attacks truly deal 2 attack in reality, with poison, he reduce more HP, depends. He also attack the Protagonist during the action menu, bones all over the actions, including the flavor text/Hit.