Using a gravitation potion makes this trophy trivial, but if you don't have any you can use blocks to climb to the top of the world. you can use minions such as the imp to handle any harpies that come your way, and if you don't have the cobalt shield then you will fall off to your death unless you have climbing claws, a lucky horseshoe, a grappling hook, or a feather falling potion. you can grapple to any block to negate knockback, but you cannot build upwards using this strategy. But for the sake of simplicity, just use a gravitation potion.
More on the gravitation potion here:https://terraria.gamepedia.com/Gravitation_Potion