dmc3se - dmc3
easy - not available
normal - easy
hard - normal
very hard - hard
dmd - dmd
hoh - hoh
so if this your first time playing it, you are playing on wussy-ass dificulties since hard isn't unlocked by default
also, agni/rudra are fire/wind, not fire/ice, and they're definitely some of the easiest bosses on all dificulties. they're only somewhat challenging if you kill one and leave the other with a load of health, since the fused sword's attacks have a big aoe and deal more damage. but you're not supposed to do that.
if i were you, i'd play dmc3, shin onimusha and then ninja gaiden sigma. heavenly sword

as for the first boss, if it's that thing that looks like death, the trick is to keep moving - preferably with trickster's dash and jumping - while he's teleporting. when you attack him, rush with stinger and keep attacking until he swings his scythe, then dodge, rinse and repeat.