Poison Arrow is a heavy amphibious tank with tons of hitpoints and powerful AoE plasma cannon. It can sneak in through water to enemy territory and cause heavy damage to anything on its way. Its gun one-shoots all T1 units and can even deal with some T2 troops and defenses. However moves and fires at a very slow rate, what makes it susceptible to fast infantry like pyros. Try to combine it with Crocs some amphibious AA like Crashers, to make it a self-sufficient strike force. Amphibious tanks can also climb hills better.