Before getting into dungeons (on any of the melee damage dealer classes), go back over the descriptions of each of your skills, paying particular attention to ones that have bonus damage when attacking from a particular direction. Since getting those positional bonuses is rarely possible while soloing, a lot of players disregard that aspect when they first get the skills and then forget about it by the time they get into a group. In a group, you'll hopefully have a tank keeping the mobs stationary enough that you have the freedom to move around them to wherever you need to be for those positional bonuses. They finally become useful, so you'll want to learn which attacks are best from which direction.


Oh, and if you haven't yet, you need to learn to recognize when a mob is readying an Area of Effect attack, and get into the habit of always moving out of the way of them. For some attacks this matters more than for others, but it's never a good idea to stand in that attack range if you don't have to. In most cases, the area is shown by a reddish orange mark on the floor. (A few primals and bosses have specialized markers to show their attacks' area, rather than the default orange glow. When they do, it's generally something pretty obvious, and you won't have to worry about that in Sastasha, anyway. I think the first time you see a specialized AoE marker rather than the default one is when you fight the primal Ifrit at level 20.)

If you have your visual effects on their default (showing "All" effects for everyone), the screen can sometimes get obscured by so many flashing flaming effects that it becomes hard to see those markers on the ground, so I recommend going into Character Settings and lowing the visual effects at least for other members of your party to "Limited" or "None" rather than "All".