GP match up is very very bad

For some advices, I encourage you to try and play Conditioning instead of Bone Plating or Second Wind on Rammus since resists are what he scales from and boosts. (also for Darius Second Wind way better if you don't take Conditioning because it procs it for each bleed and in contrario, bleed can easily use your Bone Plating and will last much longer and it doesn't reduce true dmg from his ult that you receive)
You can inspire yourself from my match-ups but I made 'em late at night when I was starting to learn every single champ, so for example Sett won't be anymore problem post 2-3 items since you will take less damage than him but he won't be able to take enough damage to stack grit, also you can Q into him to make the center deflect from your allies for example.
(btw +1 vote, Hullbreaker is promising with Demolish)
PS : When I tested Rammus top, I was proxying a lot since I was versus some hard opponents and turns out it's very effective when you buy Bramble and even before, you could try to do Shenanigans like proxy runes that theBausffs makes or whatever depending on the match-up, I won't be deepsearching into this so all your choice