"Fox McCloud vs. Bucky O'Hare" has Fox strangling Bucky and then blasting his head off with his laser pistol.
"Princess Peach vs. Princess Zelda" ends with Zelda getting kicked in the face so hard that her head splatters like a melon.
"Harry Potter vs. Luke Skywalker" has Harry screaming in agony as his head splits open at the scar.
The "Starter Pokemon Battle Royale" has Venusaur being roasted alive and Charizard getting his wings ripped off and being filled with water until he explodes.
"Beast vs. Goliath" ends with the former being graphically bifurcated.
The "Goku vs. Superman" rematch ends with Goku getting his brain vaporised by Superman's heat vision.
"Yang Xiao Long vs. Tifa Lockhart" ends with the former finishing her opponent off with a shotgun-gauntlet Recoil BoostedNeck Snap. Notable as Barbara Dunkelman, Yang's voice actress who was watching the episode's live premiere, visibly flinched in horror at the deathblow.
But even that is topped by "Natsu vs. Ace", where we get a close up of Ace's face boiling away from Natsu's lightning attack. It's not instant, either — we see his skin blister and boil away, then his eyes drop out of their sockets before melting, all the while when Ace is screaming. The Nightmare Fuel page for Death Battle even provides the sweet looking picture of it.
Happy Tree Friends is devoted to this. Every episode features deaths that are extremely gruesome and photorealistic...and yet it looks like a children's cartoon.
There are loads in Rise of the Mushroom Kingdom. Among the highlights are a Koopa burning alive in lava, Mouser's getting eviscerated, Luigi getting torn in half and......Well.....Any time a Yoshi is onscreen.
In Power Star, any time Mario pulls that sword out of nowhere, everything dies in painful ways. And those are the mooks, things get freaking violent when the main characters start dying.
In Super Mario Bros. Z, Mecha Sonic loves killing his opponents in the most painful and brutal ways possible. Just ask Sonic's friends, the Koopa Bros, and especially the Axem Rangers X.