Note: Collecting 50 treasures in any land is a significant achievement (it isn't called "Master of Ice" for nothing), and you will need to have a strong understanding of combat and the Icy Land in order to succeed here. If you are struggling to approach this number before dying, it is recommended you try a different area and come back later.
To earn this achievement, you will need to collect 50 Ice Diamonds (found in the Icy Land) in a single game. I explained the full mechanics of the Icy Land in my guide to Yeti Master here:
The Yeti Master achievement in HyperRogue worth 12 pointsCollect 25 Ice Diamonds. (or 50 in the Orb Strategy mode)If you are unfamiliar with these mechanics or need a refresher then I advise you reread that guide, but the most important thing for this achievement is knowing how to defeat a large number of enemies without getting yourself surrounded. The key to this is in using chokepoints to funnel the enemies, and in using orbs to escape when necessary. Be warned: Ice Walls will melt if you remain near them for too long, so it will be difficult for you to use a single chokepoint consistently. The Orb of Flash is a good way to kill a lot of enemies in a single turn, but be careful not to activate it prematurely or you may be left in a difficult situation. The Orb of Winter, on the other hand, is a good way to escape as you will confuse the Icewolf, but be careful as the Yeti will still track you, and you will need to create walls with the orb's power to evade them.
There are a couple of other techniques you can use to make this achievement easier. The first is that every Yeti and Icewolf you kill in this land will slightly increase the rate of treasure spawning, so you can run around for a while and kill many monsters at a safe level to cause Ice Diamonds to appear more frequently. The second is that you can bring in orbs from other lands, and if you collect 25 (or more) treasures in that land its orb will appear in the Icy Land as well for you to collect and use. If you have an orb in mind and you are able to collect 25 treasures in its land then this is a good way to increase your power and make the Icy Land safer, but know that this will increase setup time for your run dramatically (especially as the Icy Land is the starting area for the game). You can use the world overview screen (press 'o') to view the full list of lands and orbs and see if there are any that appeal to you as this is highly dependent on your own skills and playstyle. This is not an easy achievement to earn, so good luck!