White noise. Or supersaws if they're appropriate.
But really, most producers cut everything above 18k anyway, since most people won't be able to hear it and it just takes up headroom. Unless your mixes sound thin without anything in the 15k-18k band, and I do mean sound, it's not that important; just put some noise on it (high-passing it appropriately), turn it to 0 and bring it up until the mix sounds okay, and forget it. A spectrum doesn't tell you how to mix; your ears tell you how to mix.

Disable the limiter/compressor that's probably on your master, that should fix it.

If it was compressed, an EQ might shift harmonics out of phase with each other, "undoing" the compression.

Personally, I do not think bigger is better(no pun intended).  I do not know what genre you play, but you need to limit your choices when you prepare your sets, rather than bringing 50.000 tracks.  It´s no good spending the whole night scrolling for the next track.  If you want to make good sets, you should approach the gig like djs used to in the vinyl days.  If they chose more than 200 tracks(2 crates), they would not be able to bring them all.  Look at Mat when he spins.  He´s bringing 100 or so cds.

I disagree drastically. Depending on your technique, you might be throwing down a lot of tracks every set, and not all of them go well together. If you match key and tempo, and switch tracks quickly, a few thousand tracks might be necessary if you don't want to get bored every night.

A solid understanding of everything makes you good, being exceptional at one thing makes you great. Noisia are gods of sound design, but I don't really feel like listening to them casually because of their composition. Similarly, most Monstercat artists have good sound design, clear production, great songwriting, but no real forte, and a lot of them are fairly interchangeable. So there's your answer... the "T shaped skills".

I've heard some people bus the kick and sub instead of sidechaining, so it acts more like glue, but that saps the kick a bit. It could work with a little attack on the sub, though.

Saw the title and thought it was going to be about the more "acoustic" type kicks that have been getting popular lately, like this.

Oh hey, guess what? I asked myself literally this exact question a few days ago.
First: the "stutter" comes naturally when you produce the way he does. Seven Lions has said that he bounces everything, and mixes the track in a separate project. It's pretty easy to just chop up clips and either gate them, or use the brush tool to repeat a short section of sound really fast. The part at 2:43 sounds like an LFO slowing down, mapped to the volume on a few inserts.
Anyway, a lot of his basses are FM. The growl in "December" (second half of the drop), I'm 90% sure it's Sytrus. 2:28 sounds like a basic bandpass wobble though. 2:31 could be Serum; same thing, but with a squared wavetable.

What big mistakes have you seen novice producers make often, and what tips do you have to help them improve?

This thread is a followup to my (wildly successful) first question thread in which I asked "What are the top three tips that you'd give to a producer with less experience?" This thread managed to receive more than fifty different responses that all offered great insight and advice. If you would like to check out this thread, you may do so here.

I'd love to see what expertise you guys have in store this time. Thanks!

THAT LIMITER ON THE MASTER IS YOUR ENEMY.

Some default FL projects start with a limiter on the master, with the gain inexplicably cranked to 5dB. That might be why so many tracks have insane amounts of peaking. If it's there, turn it off.