The silver lizals in that room can be tricky if you're not prepared. A good strategy is to separate them; I find a spin attack to send one flying into the water and then luring the other one along the bridge away from the camp is the best way to do this. Then, use a fast hitting weapon like a sword or spear to hammer it, BUT make sure you stop the combo before the last hit so it doesn't go flying. If you do that right, you can keep up a constant barrage of attacks and it'll be really hard for it to slip away and heal. Eating a 30 minute triple attack boost (achieved by adding a dragon material to the recipe) before you go into the trial reduces your margin for error, too. Headshots and just bow attacks in general are also a good way to keep its regeneration in check if it does manage to get the slip on you, and Stasis+ is another viable option to close that gap.
