However, I managed to get my hands on 2 basic and 2 intermediate dimension coordinates, which you might have seen in my Dimensions video series:
And after completing these 4 I managed to get my Dark Resistance up to 624 without having a Hades sylph. Here below you can see my resistances at the time (left) and the power-ups given by the dimensions (right).
Depending on the difficulty you need at least 5 basics of that same element, but otherwise I'd say a minimum of 3 basics to get those resistances increased and then as the energy costs is down to 10 per step then opening 1-2 intermediate dimensions and if possible, for the 5th one, an advanced dimension. Then you will have a huge surge of resistance allowing you to beat the Sky Trail boss.
Repeating this strategy over time you can pass a lot of Sky Trail levels bringing you also a much better Horns income every day.