Totally depends if you have knowledge about the encounters or going blind.
If you know the deep dweller fight because you've done it before you can do a lot of cheese to make it a breeze.
I accidentally stumbled upon that fight on my solo tactician blind pt and it looked kinda scary to me right after Fort Joy (level 6 I think) so I used flight and sneak to get out of it.
I further explored the map but very shortly after I was up on the bridge exploring, when all of a sudden the deep dweller fight started again because I previously triggered the encounter but instead of reloading to avoid it I sneaked out.
Now because i was on the bridge I not only had the height advantage but all enemies except 1 archer could not attack me but had to waste several turns to get to me.
On top of that each time it was the deep dwellers turn it didn't even try to get to me (even tho it can fly) but just said his line each turn (which got kinda annoying towards the middle of the fight :P) so at the end it was me 1v1 the deep dweller.
But if you have no previous knowledge about a fights mechanics and don't try to cheese with terrain too much staying grouped up is really smart splitting up only really helps if you know a certain placement gives you an advantage.
Late game it doesn't matter at all since you will oneshot all enemies anyway and have giant AoE abilities.
