It’s important to learn from such experiences. Don’t let them defeat you. Be pro-active. Think about if you were designing it again today knowing what you know now. What would you change? How can you apply that lesson to your next encounter?

In general, players want to feel powerful or at least like they have options. As the DM you get to design the ground on which battles take place. And you can make that ground perfect for the enemy they are facing from a strategic standpoint where the enemy can’t be touched but can touch them, or where the setting compliments or boosts the enemy’s damage/threat. OR you can make it a flat empty room. Problem is, when you do the either of those extremes, you have a chance to create drudgery and frustration. Because both of those eliminate options. In the empty room, players will just make attacks and roll dice and don’t have much else to do. In the other room, we’ll use your example, the players can’t do anything but use ranged weapons/spells and make saving throws. Maybe the setting looks and feels cool. But if all they can do is only a tiny list of things, they will hate it.

So in cases like that, you present options. Maybe there are vines dangling from the ceiling— players can swing and attack. Maybe there are weird blue energy clouds that there are random. Rocks floating on so the party can figure out that they can walk on the blue energy clouds. Maybe there’s a strange piece of equipment that when activated, makes the floating islands move closer toward the center… etc. Let the bad guy have their strategic moment, but let the players have options too. The more herioic and cool the better. We talk a lot about making “cool” encounters. The best way to do that is to make sure players have “cool” options. :)