Martian Officer

The Martian Officer is a Hardmode, post-Golem enemy that spawns during the Martian Madness event. It will always initially spawn with a protective Bubble Shield, which provides the Officer significantly added defense and hit points, as well as immunity to lava and knockback, until it is depleted. After the Bubble Shield is destroyed, the Officer can now actually be harmed. If the Martian Officer is not killed within three seconds after the Bubble Shield is depleted, the Bubble Shield will re-activate with full strength. The Officer's health will not be restored.

Trivia

 * The Bubble Shield is one of the few "enemies" that does not have an Expert Mode buff or boost.