Where can u find metallo
You can find metallo in the Azid Ziggurat southwest of Lassandal.
I looked and can’t find it there
It’s there, it’s pretty rare, so sometime you have to enter the cave multiple times before you get one.
The spawn rate of Metallo is very low so u have to go in and out the cave if u dont find it on the floors right away
Is it on all of the floors or just certain ones?
I am pretty sure it is on all levels just really rare. I’ve had my best look on forth and fifth floor tho so maybe try there. Use checkpoint to warp to floor 4 fight everything in there move onto fifth leave and repeat. Took me about 40 minutes to find two. Just make sure you have something like stormfox that can stun it and dont attck other mobs because it can block attacked, I killed two this way 
just keep trying and never give up. 