I searched for "Strand of the Ancients" and read every thread, nothing like this came up so here it goes:
When you are defending before the battle starts at each of the depots there will be one tank spawned. Take it down to 5%, but DONT KILL IT or another one will spawn, this way when they take it it will be one shottable and they will have to wait for another tank to spawn.
EDIT: THESE are the tanks I'm talking about, not the ones on the beach
Hope this helps someone, have fun.