Thanks for the information and the link, I think I'll go for the polygon hitbox approach where the Z axis is ignored in collision detection as you suggest.

Thanks for the comment. With regards to having the smaller hitbox, I did think about this, but it would not work with what I am aiming for. I've added some extra information about this to my original question, sorry about the confusion. With regards to the circle-poly collision, this may cover what I need, but I am concerned that it would not meet the precision I would like, for example, if one enemy type is tall and thin, I would like a bullet to miss them by flying parallel to the enemy, which a circle hitbox would give a false positive to (potentially)