Larian Studios Forums Divinity: Original Sin 2 Divinity - Original Sin 2 - General Is running only a single healer viable?

You can certainly have a Hydro/Summoner character. However, I would not recommend running a single healer, because combat is designed to be challenging, and if your healer goes down, you'll be screwed. It's better for multiple people to have at least some healing or armor restoration magic.

Tanks are not recommended: the AI prefers to target squishy, low-armored characters and bypass tanks. Just letting you know that if your concept is to have a big tough guy up front which all the enemies will crowd around while your healer is safe in the back keeping them alive... that's not likely to work that well. A melee combatant which takes one level of Warfare and Hydro can combine a spellbook from both those skills to create the Cleanse Wounds skill, which is a very solid (but low-range) healing spell. Bow/Crossbow specialists can get the medium range First Aid from the Huntsman skill set.

In terms of damage, you want a balanced party, either all Physical (or all magic) damage or 2 physical/2 magic damage dealers. Summoners can cover both physical/magic damage, but I'd recommend taking at least one damage spell with your Hydro/Summoner. If you're only hitting an enemy's magic armor with one character, it will feel ineffective when multiple physical attackers are taking physical down faster.