War Elephants and Mahout only do 30% of their usual damage to resource building. Their preferred target is defensive buildings, they will not often attack them. Both elephants are not available as a regular troop. They are similar to Heavy Cavalry with higher hitpoints and lower damage than the troops available at the same age. Level 3 and 4 of these Mercenary type don't have the same traits as Level 1 and 2.
Flammpanzer can attack multiple target at the same time, making them a good choice to handle infantry defenders.
Tank Destroyer only attack single target but with powerful damage especially toward Tanks. In World War, they are usually chosen as diversion for Missile Silo.