Gotrek destroys an entire Black Ark, which is essentially a floating Dark Elf Fortress just with his axe and a cursed harp after being threatened by Dark Elves. His response: You are threatening a slayer? I'm taking you all with me.
Gotrek finishing off the Bloodthirster of Khorne who just thrown him aside and left him for dead. The best part is what he says in doing so. "Oi! You! I'm not finished with you yet!"
According to the Dwarfs, Felix's entire journey with Gotrek is a CMOA... Though specifically picking up the Hammer of Fate, something no human has ever done, and never will since, after invoking the names of the Dwarf Gods and throwing it at the demon, which is why the Bloodthirster turned its attention to him in the first place. He'd managed to hurt it enough to distract it from something as tasty as Gotrek.
Felix begs help from the gods, picks up an artifact weapon that only a dwarf should be able to lift, smiles at a charging Bloodthirster and hits it right between the eyes.
Gotrek, using basic physics and his long-ago earned engineering skills, escapes a near-inescapable death trap.
The entire second half of Elfslayer. Even by the standards of the series up to this point their previous adventures have nothing on their revenge against the Dark Elves and Skaven.
Gotrek beheading a Chaos Warlord who is famed for the strength of his armor like it was made of rotten cheese.
Gotrek scaling a giant, sinking his axe into its brain, then PULLING OUT ITS EYE AND RAPPELLING DOWN ITS BODY USING ITS OPTIC NERVE!
The skilled but incredibly overshadowed Felix goes berserk in Dragonslayer and stabs a dragon that is the size of a small castle repeatedly in the head. Of course, it was his sword giving him fury and skill so that it can be used to fulfill its purpose, but still.
The short story "In the Valley of Death" is one for Felix, cementing that he's not, as some might think, The Load who depends on Gotrek's skills to survive — instead, completely on his own, and implicitly before he's even met Gotrek, he survives a trip deep into monster-infested wilderness in search of a lost necromancer's tower, slaying several Wights and then escaping, alone, back to civilisation after confronting the lich residing there.