The Tank Killer is, as its name suggests, a tank killer, and the AP cannon it equips can easily pierce the protection of most vehicles. The downside is that this unit is only suitable for assault and harassment, not frontal attacks. Tank killers are lightly-armored to preserve speed.
Our Tank Killers and Hydra Cannons make for an excellent anti-unit support. You can deploy more of these units with a Tank Drop once a Palace is built.