So specifically for the grove battle part you are in, you should kite the druids into the packs of goblins which are scattered along the way when you move up from the grove proper. Use some bombs and grease bottles to slow their progress while the goblins come into your help.
I did the battle yesterday with two characters and it went smoothly.
But as you seem to struggle, you should have lae'zel or another tank in your party. Cause at the moment, a warlock, shadowheart and astarion doesn't seem like a good group for survival. You need a warrior or someone who can deal lots of aoe damage like a wizard. And you lack one of those at the moment.
Thank you, kitting them up the hill to get reinforcement worked wonderfully. I was also able to kill that vile Minthara after the battle ended also. Felt good.
Child killer druid and Mindflayer scum dealt with. The poor devil looking people had to pay the hard price.
