Rust Tyrano will begin immediately counting down from 5, doing a roar that inflicts HP Down every time. When it reaches 0, it will attack your entire party with heavy Fire damage (unlike Black Tyrano, this element is not randomized; it will always be Fire). After that, it will chew on your characters and absorb some HP for a few turns, then boost its fire damage output, and begin its countdown again.

The damage from Rust Tyrano's fire breath can be high. Since its attacks otherwise are minimal and its defenses average, the strategy is straightforward: deal as much damage as you can quickly. Alternately, equipment that absorbs Fire damage or Magic Wall/Barriers can help mitigate the breath attack.