Some tips for the fight (some will consider this cheesing, but it's simply making use of the tools the game provides).
- Position the party on the bridge above the patrol, do not position them all in the same place as some of the gith will throw bombs at the player.
- Destroy the ladders leading up to the bridge with a bomb / caustic bomb / anything with aoe fire damage.
- Take advantage of spells that give advantage on the targets (true strike / guiding bolt / etc)
- Use potions. By this point you should have a bunch of potions of speed and poison. This is a good occasion to use wyvern poison, but make sure you have advantage on your attacks before using it, as this will maximize the damage it causes.
- If you have a rogue, consider using hide to activate sneak attack and gain advantage. You can also do this with other classes at the moment, but 1) this costs one bonus action where thieves have two, and 2) I don't like that other characters can do this and i DO consider it a cheese mechanic for other characters at the moment (rogue should be able to do this with cunning action nevertheless, so it's fine for rogues).
- cast spells that give disadvantage to the attacker or bonus AC to the defender before the fight (mirror image, etc).
- 2 of the gith have misty step and will use it to reach you. If they do, either have a high-STR character shove them off the bridge, or use gale's "Thunderwave" spell to shove them off. That should remove a SIGNIFICANT chunk of their hp (can do 40-50 damage in one fall).
- for easy damage with Gale, consider making a detour via the underdark first to get the sapphire spark and absolutely wreck everything with magic missiles. I usually don't do this and it works fine.
https://baldursgate3.wiki.fextralife.com/The+Sapphire+SparkI understand why the fight can be considered "too" difficult, but I do like it because it does allow (and, in fact, encourage) the player to use the full set of combat features available to them. I do believe it will be made much easier when lvl5 is unlocked later, since melee players will have access to a second action. With the tactics above and if you're lucky, you should be able to 3-man the fight even with lae'zel dead. Of course, if you want to maximize your chances further, don't bring Lae'zel and instead bring someone else to the fight.
EDIT - once we have difficulty options in the game it will also be made easier. The game currently does not have difficulty levels and I think this is a great way of satisfying every playstyle without compromising on features or fight mechanics.
