I got the game yesterday and played it a lot, but I've only really
unlocked most of the monsters rather than unlocked and leveled them up. I haven't even seen Gigan yet, which is really killing me because you need Gigan parts to get the second temperature gauge with each monster. I guess maybe he appears if you play as monsters other than Godzilla? I've been through the main destruction mode a few times now with Heisei Godzilla to unlock all the stage bases for the diorama mode and get all the photos on each level, but even though I've unlocked all of those there are still one or two areas I haven't done yet that have two monster icons on them instead of one, so hopefully Gigan might show up there.
So far the only other monster I've played through it with is Showa Godzilla, and I can't be the only one who thinks his atomic breath sucks, right?
It's range is incredibly short and it does barely any damage. It's terrible. I used it on one of those big battleships that can be destroyed by one blast of the Heisei breath, and it took three or four hits without exploding, so I just left it.
I think I still need Battra adult, Burning Godzilla, Destroyah and Gigan to unlock, but it's going to take me a long time to level any of them up (though does anyone else have more question marks left than characters to unlock? I'm pretty sure those are the only four characters I still need to get, yet I have about 6-8 question marks left for some reason.).
Oh yeah, and I love that you can sometimes knock Mechagodzilla 2's head off.
It seems like you can do it if you grapple with him when he's at low health, which makes sense since that's the closest thing to ripping his head off that you can get in the game. I'd read about it before, but seeing it for the first time when I was fighting him brought a smile to my face.