There are a lot of exotic weapon and armor pieces in Destiny 2 that have been disabled due to a game-breaking bug related to them, and Telesto is on the top of that list. Telesto is an exotic fusion rifle that was added originally in Destiny 1. And since then, it has been breaking the game in any way possible. PvE activities like Blind Well, which was added in Forsaken, are one of the prime examples of glitches that include Telesto.
These glitch has been in the game for quite some time now. But now, players have found a way to use it inside the final boss room in the Fallen S.A.B.E.R. strike. Along with that, there's also a cheese spot in the boss room of the same strike. When you drop down in the final room, there are two pillars on your left and right. You can actually hide in these pillars. This is useful if you're the last person alive in your Fireteam.
This cheese spot will work for the majority of the boss fight until the final Arc phase. In the Arc phase, the boss starts to roam around and follows you doing arc damage. Thanks to Cheese Forever, we now know a new spot in this boss encounter that is similar to The Arms Dealer bubble cheese.
To do this, you need to jump into a small tunnel found at the tip of the pit in the middle. There are two ways to enter this tunnel. You can either enter it from the front which is already open. Or, you can shoot the panel at the other end of the tunnel which will open a more safe way to enter. However, you will take Arc damage while in the tunnel, so using something like Well of Radiance or Bannersheid Sentinel Titan here.
When you're at the edge of this tunnel, you can use Telesto to damage the boss. You can also use Solar or Vortex grenades here which will do damage to the boss.
Source: YouTube