So every year we typically get this sort of thread popping up, so I figured why not start it earlier rather than later! Share your Dark Harvest Festival tips and tricks for getting those tasty candies!
One tip that I recommend that I don't see very often is to not use multi-hit attacks on the Punkin King head. The head will only drop one candy per attack, and five candies in total (I believe, that number may be smaller in earlier tiers). However, the multi-hit attacks will only drop one candy while also taking away from the total candies dropped, meaning if you brandish charge the punkin king head you're actually losing candy.
To avoid this, hit the head 5 times with a regular attack, and then from there you can feel free to use your powerful multi-hit brandish charges.
Adding to this, instant-killing the early tier punkin heads doesn't let drop its full candy amount. Try to hit it a few times even if it takes a tiny bit longer to chase him down!
So I already have the three outta four authentic masks, these being the Phantom, Frankenzom, and Spookat.
A really easy way to get these is to find a low-level general clockwork level (Power complex, wasteworks, clockwork tunnels, etc) and spam it on normal mode, avoid all monsters and just rush through till you find the event room. Re-start the level the moment you find the event room with the Punkin king and collect your tokens, or if you can't find the event room, just re-start. You'll save on time and get 3 of the four candy tokens, Sweet, Sour and Hard.
As for Brittle candy, it appear these can mainly be found by Grim Scarabs/Gourdlings. These monsters, as pointed out by another user (Flowchart), spawn in the graveyard levels. The best level to do this is by grinding in the first floor of rank 9-1, An Occurrence at Owlite Keep. Check every grave for Grim Scarabs, and ignore the Zombies and Phantoms if possible. Once you've checked each grave for Scarabs and have defeated all the ones you can find, re-start the level till you have all the tokens you need to trade in for your authentic Scarab mask.
The first method is also a great way to farm for Prizeboxes, Punkin sprouts, etc, as you can trade in any token for these materials, and since Sweet, sour and Hard candies are easier to get in large quantities, it's a much more effecient means of getting what you want quickly.