Elements should work against bosses - well, there's a challenge for clearing each boss battle using the element they are weak to so one would assume it should work... but I've been doing my run using mainly just the default buster so I've not touched on that yet.
However if you want to deal nice damage I found upping the bullet size to large, using the fire shot (not sure if others will work better?) and then enabling the option for bullets to break apart on impact. That deals out extra damage on enemies and especially bosses (just watch the health drain away!). Add in a nice defensive boost and it basically feels like your invincible (kinda breaks the game for me, good for S rank runs though!)
Almost done with the 9 initial stages, so far I think the Radio Tower (Mighty No 6) has the worst level design of the bunch. Far too many instant death points where a single mistake fucks you over. The boss fight has a pretty steep learning curve to it initially as well (but once you get him down its actually pretty fun). The highway (MN7) is my favourite stage/boss so far, nice and fast paced with a great boss waiting for you at the end of the stage.
MN2 is still the worst boss fight I've encountered so far, she's almost as bad as she was in Mighty No 9 (but not quite), why they kept that stupid freeze gimmick I'll never know, she can basically stand right on top of you and you can't do an awful lot about it...
So overall I'm loving the game bar a couple of issues with the initial learning curve on bosses/levels. Kinda hope this sees at least a limited physical release in Japan as I'd totally buy a physical copy to go with the rest of my Mega Man collection.