Divide and conquer

Relies on your teammates being cooperative.

Works especially well on maps with few key points like mansion.

Step 1

Grow your eco as much as you can, and start researching artillery(if the enemy is bothering you with troop movements, make sniper jeeps) ask teammates to research Juggernauts to help

Once artillery done continue to next step

Step 2

Prepare your army.

(if you have time, and enemies are still relatively weak)

2x Humvee + 4 Juggernauts 2 medics

(Or one with 4 juggernauts  and 2 snipers)

3+ Artillery

Heavy tanks

(if they have a pretty large army)

1x Jeep + 3 snipers + 1 medic

1x Jeep + 2 snipers + 2 medics

3x artillery

Heavy tanks

If the map has water use battleships/destroyers to delay/prevent enemies from bringing support

Get one of your teammates to hold any exposed positions you are leaving bare

Move troops to the strongest base they have.

Send sniper jeeps in to clear out any troops that might prevent your artillery from getting in range(keep it nearby if there are troop generating buildings present).

Set artillery to start hitting their base, get your teammate to hold off any enemies

That might try to break the siege.

Once all defensive buildings are broken, rush your sniper jeeps in and destroy the base.

Once done, get the defending teammate to use some troops to clear the players buildings/troops while you and your other teammate heal up and replenish any losses.

Step 3

Get defending teammate to attack opponents weakest base to force them to go protect it.

While they are going to protect the base, rush into the strongest base there is(same tactics as last time).

At last, rush last base and just simply steamroll their base.