Welcome to the forum
On Silkroad-R, if you just upgrade one of the 3 elements (fire, light, cold), you'll get them all.
So really, just upgrade the Heuksal and Force masteries.
As for which specific skills within fire, light, cold, and Heuksal to upgrade, just look at some of the older guides. They are still correct (for the most part).
Essentially, you need the damage increase skills from Fire and Light, the defense buffs from Fire and Cold, and the fire imbue.
Fire away if you have other questions - I know this answer was a bit short.
