#You can't, you want *phantasmal force*

As you've noted, any physical interaction with the image (fighting it) would result in learning that it's an illusion. It doesn't matter how gullible they are, they're interacting with it so it's going to become an illusion.

What you need to do is use a spell that convinces the target that it's real. I'd recommend the following:

*[Phantasmal Force][1]* - 2nd level
>While a target is affected by the spell, the target treats the phantasm as if it were real. The target rationalizes any illogical outcomes from interacting with the phantasm. For example, a target attempting to walk across a phantasmal bridge that spans a chasm falls once it steps onto the bridge. If the target survives the fall, it still believes that the bridge exists and comes up with some other explanation for its fall - it was pushed, it slipped, or a strong wind might have knocked it off.

A gullible target still has an opportunity to use an Investigation ability check, but if they're gullible, then a DM could rule that they'll just keep fighting the creature thinking it's real and never require the ability check.

Other won't see what they're fighting, but the target won't care. They'll keep on keepin' on and fight the good fight against the illusory creature.


  [1]: https://www.dndbeyond.com/spells/phantasmal-force
  [2]: https://www.dndbeyond.com/spells/phantasmal-killer