I learned something about Master Yi vs. Rammus today.
Do not build Wit's End against rammus. It is absolute garbage now. I tried to 1v1 a rammus with yi with skirmishers sabre: bloodrazor, BOTRK, and wit's end and tier 1 boots and he had a Sunfire and a Thornmail. He ended up destroying me. In Cowsep's highlight video, https://www.youtube.com/watch?v=Iz5czZTv48k&t=46s he 1v1s a rammus at 33 seconds in and he wins. He used Skirmishers sabre: bloodrazor, BOTRK, Maw, and Merc Treads.
TLDR: instead of getting wit's end on yi vs rammus, get a Maw instead.