Build the boring border first
Straighten your frontline, abandon exposed bulges, and use defense posts on the borders where two players can hit you at once. A compact shape is easier to hold than a larger messy one.
Do not empty the border against an ally. In FrontWars.io, an alliance signal is a temporary shared interest, not a permanent contract.
Bait impatience
Many teamers overattack because they expect the solo player to collapse. Keep enough reserve, let them spend troops into fortified ground, and counter only when their bar drops.
If two neighbors are both attacking you, send diplomacy signals to the weaker one while defending the stronger one. You want their attention split.
Turn third parties into pressure
The strongest anti-teamer tactic is not winning a 1v2 directly. It is surviving long enough for another player to attack the biggest threat from behind.
When the team starts fighting elsewhere, use a controlled 40 to 50 percent attack to take the broken border, not a full send.