Hearthstone Mercenaries is a game about strategically combining units and taking advantage of their synergies. When it comes to the double damage system, inspiration is clearly drawn from how combat works in Pokemon RPGs or any other game with a power triangle. Simply put, this particular Hearthstone Mercenaries mechanic is like Pokemon typing: a rock-paper-scissors system in which every unit type deals double damage to one of the other unit types, and receives double damage from the third.
In Hearthstone Mercenaries, all units are classed as either Protector (Red), Caster (Blue), or Fighter (Green). Protectors deal double damage to Fighters, who deal double damage to Casters, who themselves deal double damage to Protectors. An easy mnemonic you can use to remember this system is to think of each type as a classical element: fire burns grass, grass sucks up water, and water extinguishes fire.
Hearthstone Mercenaries double damage teams
The general tip we can give you about building your teams in Hearthstone Mercenaries would be to include at least one member of each type, in order to always have the option to deal double damage against specific enemy units. This is not bad advice, and it will probably work more often than not in PvE runs. A safe team composition with two protectors, two Casters, and two Fighters makes a good jump-off point for your first few games.
That said, the double damage triangle is only half of the team-building meta in Hearthstone Mercenaries, and as is the case in most PvP games, you should not focus on what seems to be a good idea on paper. Instead, try to experiment with the full range of tactical options the game offers. A good synergy between units’ unique abilities can easily do more for you than the simple double damage buff, especially in PvP which will soon be a predictable string of encounters against meta teams and units.