You can't do any stealth there, as soon as you go into reach the dialog and afterwards the fight begins. If "git gud" means placing oil barrels and abusing fog cloud bugs then I won't "git gud" at all.
I don't expect to win every fight, but this fight is NOT winnable without cheesing or multiple reloads, so there should be at least either a hint that those giths are so strong (even if I would expect them to attack me, I wouldn't expect them all to be at level 5) or a way to avoid the fight.
I am no newbie in RPGs, I play DnD for a long time and if my DM would challange my level 3 party with such a fight he would just be.. a bad DM. A good DM would give you hints that if you go this way, you would most likely die.
Ok... as a DM I agree you as a player should get an hint about these Giths being dangerous psycopaths. Fact is you get all those hints. Lae'zel doesn't know what she's talking about (listen to her banter carefully and you'll get it). The Githyanki are a bunch of evil zealots and them talking about "purification" should raise alarms in your mind. Third, when you notice the adult red dragon you should know they are pretty dangerous.
Anyway: try unlinking your party, place them on the towers above the battlefield (you can easily do so when approaching the Githyanki, it's the spot where the tiefling huntress is hiding then spread your characters out) then send your tank to talk with the Gith and start the encounter. When they try to engage you, hit them and then shove them down, the combination of attack damage plus falling damage will get rid of most of them. If you need to finish them with bombs or fire/explosives comboes.