Synergy is roles that help each other. A board of one role breaks.
The core pair
Melee holds the front; ranged and magic deal damage behind it. That pair is the base every board scales from. Build it before anything fancy.
Support multiplies
Support units behind the pair boost what it does. A board with melee, ranged and support clears more than the same gold spent on three melee.
The cancel
Two melee with no ranged behind them, or support with nothing to boost, wastes gold and lanes. A role with no partner is dead weight on the board.
Build the pair
Take units that fit the pair the next wave needs. A merged spike into the pair multiplies; a merged spike into a solo role does not. Synergy is the multiplier.
The unit guide lists each bonker’s role, so you can see which pairs your shop offers.
Combine roles, not just copies
Synergy is when two paired roles boost each other — a tank that taunts plus a dps that focuses the taunted target, or an economy bonker that funds a spike merge. The shop offers pairs; buy the pair the unit guide says combines, then merge them. A synergy pair multiplies the lane’s output; two unrelated units just occupy slots.
Read the shop for pairs
Each shop refresh, check which roles you can complete. If you hold a tank, buy its dps partner; if you hold economy, buy its scaling unit. The unit guide’s role column shows which pairs your shop offers, so you build the synergy instead of grabbing whatever is cheapest.