Some general tips:
First and foremost, get to level 4. This is a tough fight and being two levels down is a _huge_ handicap to overcome. I did this battle at lvl. 4 and only had a bit of trouble.
Remember to bless your damage dealers. That extra d4 to attack might sound like a little, but adding 5-20% to your hits can decrease your miss chance by 50% (you should have a ~40% miss chance without advantage, so getting that down to 20% would be a 1/2 reduction in miss chance).
By the same token, remember to Bane your enemies.
Cast mage armor on Gale since he is squishy and will be focused by their ranged damage dealers. If you can, give him temporary hit points as well.
If you have the Sword of Tyr (or whatever it's called) from the quest at the tollhouse, use that to give someone an extra 2 AC.
If you've completed the goblin horde quest and have the Absolute's Shield (or whatever it's called), you can use its power to give someone Bane (not sure if this is a bug or intended since the text implies bless rather than bane). Put the shield on ShadowHeart since she can't do much with her bonus actions, and then use the shield's Bane on their ranged archer if you have the chance.
Drinking a potion of invisibility only takes a bonus action. If you have some, use them.
In this specific battle, the ranged Gith (two of them, I believe) will Misty Step and move up to the bridge to get advantage. This makes it very easy for you to shove them off for massive damage. It also means that you can seed that area with explosive barrels so you can insta-kill them. Or pre-position a melee character there. Lots of options when you know where your enemies are going.
That leaves the two melee-oriented Gith to deal with. They're actually quite easy, but you should focus on them one at a time. Your rogue PC should focus on melee DPS since dual-wield backstab sneak attack is *great* melee DPS. You should have +1 weapons by this point so a +3 dex should get you something like 20 damage a turn. IIRC, 2d6+2 from 2x shortsword+1, 2d6 from sneak attack, 3 from dex -> 19 avg damage assuming you hit (action + bonus action). Gale can do Magic Missile for guaranteed 3d4+3 damage (never underestimate guaranteed avg. 10.5 dmg; more if you have the amulet from the underdark that adds psychic damage). Lae'Zel can attack either with a Greatsword+1 (IIRC, 2d6+1+3 for avg. 11 dmg) or have her cast Magic Missile as well if she's an Eldritch Knight. With ShadowHeart you can do lvl. 2 Inflict Wounds, dealing 4d10 -> avg. 22 damage. Averaging 60 dmg a turn from your party, you can 1-shot either the rogue or the fighter, or get them down low enough that you can Sleep and run. Instead of Gale doing Magic Missile, you can try Scorching Ray which does 6d6 -> avg. 21 damage but I tend to find the miss chance not being worth it unless you've got ~90% on the attack (~70% all three rays hit; ~95% 2+ hit). But, long story short, you have enough DPS to one-shot an enemy every turn if you focus fire and get a bit lucky.
The fight shouldn't last long enough to burn through all of your spells, assuming you're fully rested, so go wild with your slots.
Sometimes, the ranged Gith will go into a debuff loop and focus on your weakest character. This is good since their debuffs do less damage than their actual attacks and sleep+entangle can both be remedied by a single Help action. If you successfully one-shot one of the enemies in the first turn, then they only have 3 actions while you have 4. If they waste 2 actions to waste two of your actions (debuffed character + helping character), then they have 1 worrying action vs. your 2 remaining actions, probably giving you a big DPS advantage. Or, you can let one of your characters go down and pursue an all-out aggression strategy where you out-DPS your opponents. Remember that your characters have death saves whereas your enemies simply die. And death saves can soak up lots of damage if your enemies attack them. And a revived companion with 1 HP can still take an attack to the face and save your useful party members the damage.
