Monster one is pretty fun aswell but again no real clue with unclocking the higher tiers, boomers and the standard grunts are pretty powerful!
Yes, higher tier doesn't necessarily mean better, you gotta know when and how to use them.
Berserkers aren't all that great for getting kills, regrettably. The overhead smash move has a very annoying delay to it and what's worse, it doesn't have much range, so enemies can just roll or even walk back to evade it. What Berserkers do best is breaking shit: they can break ANY fortification, Silverbacks included, with one smash. They can also break most things just by running through them. Spawning a Berserker is ideal to break every barrier in the higher waves to let your teammates in quickly and with no hassle. That will give you a load of points and it'll significantly cut down your wave clear time.
Armored Kantus are great at offense thanks to high resistance to bullets and damage in general plus their quick movement. Dual wielded Gorgons will make heroes go down really quick at mid/short range. Since they don't have long annoying delays for melee moves you can beat down DBNO enemies quickly too. The roll attack is kinda gimmicky, I don't use it much but you can still score sweet multikills if many heroes are crawling around the same spot. As for the healing ability, it's okay for the higher waves where the heroes' damage output isn't as ridiculous, but it's a waste of time healing your mates in wave 12 since the Onyx Guards will make minced meat out of anything in sight.
The only top-tier beast I really suck with is the Savage Boomer. The Digger reload is much quicker than the Boomer's Boomshot, but I find its destructive power kinda lacking. I think it's the least useful of the top-tier beasts, but you can still score awesome multikills with it if you time your shots right. Personally I much prefer the regular Boomer.
For waves 10-12, a combination of a Berserker, a couple other top/high tier beasts and a couple Wretches is a killer. After the Berserker destroys all the barriers the Wretches can walk in like nobody's business and either pick off DBNO enemies or melee the others to death since they'll be mostly focusing fire on the Berserker. I consistently rake up loads of kills with a Wretch in wave 12 this way.
For the whole duration of the game in general, I find Wretches, Savage Drones, Boomers and Grenadiers to be the only TRULY useful types. You can dominate all waves with a Drone and make the most money consistently if you know how to bypass most barriers. There's a load of fluff monsters that are cool to play as but ultimately not too useful considering the price.
Phew, that was a long post. What can I say, I've found I enjoy Beast mode a lot more than I thought I would, haha.