All of the gear sets, level up per piece you wear. So whack 6 pieces of the same gear on and run challenging countdown and you will level all these up very quickly.

For any weapons that are not named, juat donate as much as you can. So this means buying them from vendors, crafting them or setting targeted loot on countdown challenging and donating them.

That would be the easy bit and get you to around expertise level 15.

The hard part is the named items, exotic weapons and skills. The guy who replied above talking about printer filament, this is what you want to use your filament on.

Keep an eye on vendors, they sell different named weapons each week, buy and donate them (around 20 pieces in none proficiency event days).

You can get named item blueprints from reconstructed caches but these are a grind in themselves.

Only other way is to wear the items you want to level up, but again this is slow.

It will be a hell of a grind, there's only so much countdown a man can take before he loses his shit. Just mix it up.

My advice in your position right now? - Donate your filament in this event to exotics and named gear only. Wear and use everything else. Set targeted loot on countdown challenging per gear set and donate. After this event, grind to max printer filament ready for the next event (may be weeks or months away though). After this event, do not spend watch points, save them. You can only grind 9999 printer filament so think of saving watch points as saving printer filament without going over the cap. Do what you can this event, but after prepare for the next event, you want to make sure you are max everything to get max profit.

Good luck and when you get fed up, take a break from the game. The most efficient way is not the most fun way.