The primary goal of the offense is to get through all the gates to the ancient relic in the keep at the end of the battlegrounds. The gates can only be brought down through seaforium charges and demolishers. When you are coming off the beach, take a seaforium charge if you intend on using it on the gate. This comes useful when you're going against a defensive group that doesn't like to be out further than the gate and therefore wouldn't be able to disarm the charge in time.
The defense will probably be waiting by the demolisher on the beach ready to attack it and destroy it before you have a chance to use it. Here and all throughout the game you have to make sure that opposing players on foot can't damage your demolishers, otherwise they'll never make it to the gates they have to knock down. Any players that can do slowing effects and damage from the passenger spots should join the demolishers:
The defending team will have two turrets to every gate, and it's important that if they are manned that ranged players work to destroy them without placing themselves in danger of getting shot by turrets. It's possible that once the gate is down melee players can walk around and attack from there.
Capturing the first two graveyards will enable the workshops which allow more demolishers and are a good spot to pick up new seaforium charges. If a Shadowstep or Killing Spree Rogue is on the offensive side, they can use their respective abilities to teleport themselves up behind the turrets and either assault them from there to distract the turret's pilot or run past the gate to capture the next graveyard undetected. Capturing the last graveyard will push the defensive side back to spawn in their own courtyard.
If you are having trouble getting tanks through because of players on foot destroying them, synchronize your demolishers to move in at the same time so that focus will be divided, or at least be too focused to take out every tank before they hit the final door.
Clicking the orb at the end will instantly win the round.