Learn Italian: Con questo scudo ero invincibile. - With this shield I was invincible.

This is the imperfetto ludico (playful imperfect). In Italian, the imperfect tense is uniquely used to establish roles and rules in make-believe games or imaginary scenarios. By saying ero, the speaker is setting up the fantasy: let us pretend that with this shield, I am invincible.

If it is a present make-believe game, why does the English translation say 'I was invincible'?

The translation reflects the literal past tense of ero. In English, kids also sometimes use past tenses for pretend play, like saying 'I was the cop and you were the robber'. However, if translating the intent of an ongoing game, 'With this shield I am invincible' would also be a perfectly natural English translation.

Does the ending of invincibile tell us if the speaker is male or female?

No. Adjectives ending in -e in the singular are identical for both masculine and feminine subjects. We would need more context to know the speaker's gender. To make it plural, it changes to invincibili.

Could I say sarei invincibile instead of ero invincibile?

Yes, but it changes the tone. Using the conditional sarei treats it as a standard logical hypothetical, as in 'If I had that shield, I would be invincible'. Using the imperfect ero actively creates a simulated reality or a rule for a game you are already participating in.