These things are indeed random, but sometimes random things develop into what appear to be patterns, just like a winning (or losing) streak at the casino. Also, I'd figure that patterns might also be sometimes built into the random generation, for example one of the "random" things the game might be programmed to throw at you is to repeat something several times. A few thoughts:
Are you playing solo ops? If so, are you spawning AI team mates? They ought to be able to handle the boss waves pretty much on their own with no input from you. When I began playing, it took me a while to figure out that I could spawn my team mates, and how to do it. If you're not doing solo ops, consider doing so. If you're doing solo ops by yourself with no team mates, it's gonna be extremely difficult at your skill level. Solo ops on easy difficulty is how I developed most of my initial skill with this game. I recommend you go back to easy difficulty until you improve. The difference in coins earned is not vast and the XP is the same.
I don't know how to put this part without seeming unkind: the bosses on normal difficulty are really not that hard to defeat. You need to improve your skill. I was there once myself, and for quite some time too. But I can easily finish off a super boss wave by myself, it is simply a learning curve. Aiming, utilizing cover, knowledge of how to play the various characters and variants, situational awareness, knowledge of the maps and the AI behaviors - these are all things that you will get better at with practice. I would offer to play with you and show you how to improve if you played on PS4. I recommend you seek out some garden and graveyard ops gameplay on youtube or twitch and you will pick up lots of tips and ideas just from observation.
Here's a couple practical examples: the Yeti has an icicle attack. If you are under cover, it can't hit you. Regular gargantuars pretty much can't do anything to you if you are on a rooftop. Stuff like that.
As a fellow oldster, I can tell you the learning curve of a game like this is much steeper for us than for young people. I can totally sympathize with your situation because I've been there. I used to suck with a capital S, but after a year plus a bit, I am now of moderate skill. My seven year old daughter is an ace at garden ops, though she's only played enough to be level 200 something. The only reason I get to be better than her is because she doesn't play regularly!
Lastly, you will have a much higher chance of a super boss wave if you plant an enchanted garden. So I'd avoid those if the bosses are proving too difficult to defeat. Take your time and don't be afraid to run away during a boss wave to heal up or find a more advantageous position. There is no timer and you actually earn more coins the longer the game is prolonged.
I'm going to move this thread to General Discussion so that you will get more replies.
