Spread out or stand together
Standing together focuses damage and keeps allies in reach. Standing apart keeps one ability from hitting everyone. The board across from you decides.
Checked for patch 18.4. The numbers come from the live Set 18 data and follow every patch.
The idea
Many abilities do not stop at one target. They hit every enemy within a few hexes of where they land, or they aim for the spot where the most enemies stand, or fire through the most enemies in a line. Against those, every unit standing next to your carry is one more reason for the ability to land on it.
Standing together has its own value. Units close to each other go for the same enemies and bring them down faster, and some abilities that shield, heal or strengthen allies only reach the ones within a few hexes. While enemy area abilities are weak, a tight board loses little to them. The more stars and items the enemy's area units carry, the more spreading out is worth.
So read their board for it. Find the units whose ability hits an area and how wide, in hexes, then keep your carry out of the biggest group they can hit, with a tank still in front of it. A board is only 7 hexes wide and 4 deep, so a wide area always catches someone. The point is that one hit catches the group or the carry, not both.
See it live
Your board against a current board with an area ability, the one on its most built unit. The area is lit where one hit catches the most of your units. Move your units and get your carry out of it.
Their Kennen hits every enemy within 2 hexes of where it lands.
Firestorm: Charge up, gaining 15% Ability Power per Burning enemy. Then, gain 230/300/3000 Shield for 2.5 seconds and rush through a nearby group of enemies, dealing 80/120/1500 magic damage to each. After rushing, unleash a 2 Hex firestorm that deals a total of 475/715/5000 magic damage split among all enemies hit over 2 seconds.
Placed like this, one hit can catch 7 of your 8 units, your Pebbles among them.
Tap one of your units, then an empty hex of your board, to move it.
The rule of thumb
Spread out against area damage, stand together when shields and focus fire win the fight, and never leave your carry in the biggest group.
Check yourself
Their board has a unit whose ability hits every enemy in an area. Where does your back line carry go?
Pick an answer.