So I just hatched a cyclozar in the valentine’s festival and now a confusion to use cyclozar or keep using shadowrym in my team
(I also have chromera in my team)
I’d stick to Shadowyrm, since it’s especially useful again Aegisdragon, and you already have Chromera to deal with poison/Give Turn monsters.
Cyclo is more versatile. Twice as many kill options as shadowyrm, half as likely to become a useless deadweight. And even if he does, he can just jump straight off the field whenever he wants.
Shadowyrm’s stun immunity and bloodthirst are what make it good for PvP, as well as the secret skill for taking down all protector monsters instantly. The problem is that protect/sleep killer don’t have a particularly large pool of enemies they can kill in PvP. Chrono/toxic killer have a huge pool!
I don’t have Shadowyrm but I have both Chromera and Cyclozar. Chromera is arguably the best because of roaring entrance, fast accelerate team, bloodthirst and the two best killer moves. Only problem is that it gets stuck on protector monsters. Cyclozar doesn’t get stuck on anything but toxic killer (if you unlock it) is only one-time use which is super annoying. There are quite a few thunder legendaries which are frequently used in PvP that it has to kill in 2 hits. Also, without targets it can hardly deal any damage so you are often forced to attack a particular target which you may not want to.
Out of Chromera vs Cyclozar I’d say Chromera helps me out more in PvP. Whether you pick Cyclozar over Shadowyrm probably depends on how you find your Shadowyrm. Do you often struggle to find targets for the killer moves or to charge the bloodthirst at least with one kill? If so, maybe try out Cyclozar instead. However, I bet you’ll miss the stun immunity!
I assume you’re talking about PvP here because in PvE the “killer” monsters generally don’t do that well.
P.S. Usually when an opponent sees my Cyclozar on the field it’s the first thing they try to kill. I guess it’s probably one of the most intimidating monsters out there 
Use both in your FL
I’m still pretty high on cyclo, but it all depends on your team make up. I like the fact that he comes in with pretty quick entrance speed can remove a protector then exit bringing in a stunner. And also the above mentioned if enemy has no stun protection can exit straight away bringing in stun. Also the fact his sweeping is very versatile with his chrono killer and one off toxic can come in very handy.