I have SMW for GBA, but I imagine it's not any different level-wise. I faced the Reznors, they're not all that hard. It gets very frusterating when you fail, but have patience. Now, I haven't played this for a while, but I remember what the Reznors(the rhino-like heads, right?) were. Do you not know how to defeat them?
And if you're having trouble with the lava, keep in mind to always have the cape with you. I'm sure you know this, but holding jump in the air makes you decend a lot slower. It's very helpful when crossing obstical pits.