In one fight sequence, two characters kill roughly twenty attackers using various methods. In another sequence of expository flashback scenes, one character kills 50-60 people, some for comedic effect, many gruesome, some offscreen, many onscreen including closeups of their death implements. A man getting killed with a chainsaw is seen from a distance. Blood is spraying from a distance.
Deadpool and Cable fight inside a moving truck transporting prisoners. During the altercation, a prisoner accidentally gets shot in the face, and blood spatter is seen. Cable twists Deadpool's arm and punches him, and he kicks Cable in the face. Cable shoots at Deadpool who blocks several of the bullets with his swords, but many of the bullets go through him, with blood spurts seen coming from his back. The two continue to fight with weapons, and Deadpool gets punched, kneed in the face, and hit with a car door. The truck then crashes in a MASSIVE crash that levels a freeway, and causes much destruction.
Most of Deadpool's "deaths" or limb losses are pretty graphic.
Colossus and Juggernaut have a brutal combat brawl. They violently smash each other's faces break each other's hands. Juggernaut has an electric cable inserted into his rear end and electrocuted in a pool of water.
There is a prison riot that takes place in a super max prison called The Ice Box. Inmates brutally beat each other up. Deadpool and Cable fight which results in Deadpool getting stabbed and having bones broken by Cable.
Deadpool attempts suicide multiple times, once involving blowing up his apartment with a lit gas stove and the other by drinking bleach and jumping off the roof of a building.
Deadpool throws his katana sword towards Rusty Collins and it knocks his head with its handle. No blood and gore are shown (but quite shocking).
There is one scene where Deadpools hand is impaled with a gun. This scene is very graphic and you see the insides of his hand.
In "Once Upon A Deadpool", most of the goriest violence is either cut or toned down for a PG-13 rating.