Let me guide you through getting that tricky second medallion piece - it's actually quite interesting because it unlocks one of the most challenging areas in the game. You'll want to make your way up to castle sol in the mountaintops of the giants (if you haven't reached the mountaintops yet, you'll need to progress through leyndell first).

The real challenge here is commander niall, and there's a good reason for that - he's essentially guarding access to an entire secret region. When you face him, he'll summon two spectral knights right away, and this is actually teaching you about the importance of target priority in boss fights. Take out those knights first, since dealing with all three at once can be overwhelming (frost resistance gear helps tremendously here).

Once you've defeated niall, which might take a few attempts, look for the elevator behind his arena. The medallion half sits at the top of the northeastern tower, near a ghostly figure (interesting bit of lore there, actually, as this ghost hints at the area's history). When you've got both halves - and you should already have the right piece from that disguised fellow in the albinauric village - return to the grand lift of rold. Here's something that trips up a lot of people: you need to switch actions at the prompt to use the secret medallion instead of the regular one.

This whole sequence unlocks the consecrated snowfield, which then leads to miquella's haligtree. It's worth mentioning that this area is completely optional, but it contains some of the most rewarding challenges and important story elements in the entire game.