Cathartic Denoument

Those tips in the vid work fine if you have a full team to play with, but if you're soloing, they're useless [*waste of time*].

Here's my Solo Strategy for Beast mode:
[Difficulty Level: Casual]

(Depending on your kill-speed & survivability rate)
For Tier 1, start off with the Savage Drone to get your kills in quick.
If you get killed or run out of time & fail, switch to the Butcher to take the fortifications down.
If you choose to start with the Butcher, then have the same plan & switch to the Savage Drone if you get killed/run out of time & fail.
Only reason I start with the Savage Drone first is because you start off with a short amount of time & haven't increased your time from kills & destroys yet.
Focus on kills in the early waves of each unlocked Tier, then focus on tearing the fortifications down later on.
[Also: Prioritize the fortifications. Fences (or Silverbacks, if those are already out) 1st, Turrets 2nd, & Decoys last.]

Once Tier 2 is unlocked (again, depending on your kill-speed & survivability rate), go for either the Bloodmount or the Mauler. I suggest going with the Bloodmount for the early waves of this Tier, then switch over to the Mauler on the later waves of this Tier, prior to unlocking the next Tier.
Don't forget the Bloodmount's spotting ability - thats how you get faster kills with this guy.

Once Tier 3 is unlocked, go for the Boomer for kills in the early waves (and if you're quick on getting kills, feel free to get the Savage Grenadier, though I would advise against it if your survivability rate sucks), then switch over to the Corpser for the later waves or if you find yourself overwhelmed.
Corpsers can heal by burrowing underground, but keep in mind that it takes time from potential kills & destroys.

Once Tier 4 is unlocked, feel free to try out the Armored Kantus in the first couple of waves, but after that, even the Armored Kantus won't be enough - you'll get killed by their non-bullet weapons.
Basically, once you unlock Tier 4 & the Berserker, just go all out with the Berserker for the rest of the waves 'til you finish.
And this is when prioritization of fortification destruction becomes important.
Once you have the Berserker, destroy the Silverback & turrets 1st, then proceed to BERSERKER SMASH the Gears while JUGGERNAUTing [read: sprinting] through the fences.
And if you need even more time, smash the decoys too.

I'm not sure if this same strategy can still apply to the harder difficulties, but I'm pretty confident that it still can, with some minor adapting/tweaks here & there.

And there you go! Enjoy smashing Beast Mode, Solo!