On Fjordur their eggs spawn in the back of the wyvern trench you can enter from a cave located near the wyvern trench on the side closest to the obelisk. I recommend a maewing with decent HP and Melee to enter near the start of the magmasaur cave. The inside of the magmasaur cave is very hot so i recommend ascendant ghillie (i am not even hot when wearing ghillie). Inside the cave is element shards (redish rocks near the back inside a nest and near the middle on a small side piece of land), crystal, metal and BLACK PEARLS!!! Nests spawn in the middle with the lava lake. I recommend to go inside the cave a strong tame (i go overboard and use a strong carch with a very good saddle doing 4k damage on max rage.
If you get too close to a wild one that used it's special ability (a charge that will set it on fire and aggravate nearby dinos) it'll display a message saying "getting hot! Stay away!!!!" and that will set you on fire. Don't approach unless you know you're prepared.
Use tranq arrows. It may sound dumb, but their torpor is way lower than their health. I got an egg by just knocking out three of them and teleporting away. WARNING: their torpor drains VERY fast, so you need to keep tranquilizing them while their asleep, or kill them while their knocked out.