Hydra Arrow provides a way to increase tower damage that is separate from all other damage multipliers. This makes it one of the best items against Vir on a tower with a bunch of regular stats (damage, crit, speed, multicrit).
On most towers, the effect is +40% damage over 4 seconds. On light towers, it can become +100% damage over 4 seconds.
I think the current scaling and damage amount is good for towers up to level 99. It's strong, but balanced by the time it takes for the damage to be applied.
I propose changing the damage to poison creeps for 10% damage per second that lasts for 2 seconds (+ 0.02 sec/level).