Well you're in luck as there is a new feature called Z elevation which might be useful in providing an illusion of jumping, it allows an object to change its Z so it looks like you're jumping towards the camera, or platforms are lower in the space :
The instance's elevation on the Z axis. By default the camera is at Z = 100, and looking down to Z = 0. The default Z elevation is 0. Increasing it will move it upwards (towards the camera) and decreasing it will move it downwards (away from the camera).
If you don't want to use this, you would just mimic it with an animation and scaling the character.
In terms of dodging projectiles you would have a state where player is jumping on a jump key pressed, not like in platform behaviour, for this you would have to create your own variable isJumping and toggle it on and off. Then you would say when player isJumping, disable collision on the player. This would mean that enemy projectiles no longer touch the player. You could also do it the opposite way, projectiles do damage or explode if projectiles overlapping player and player is not jumping.
As you are unsure of the design yourself I guess that makes sense why the original post is confusing. To have a 2D side scrolling beat em up combined with a top down 2D RPG doesn't make too much sense. But the information above should help you to mimic jumping from a top down view.