http://www.americanhardcoregames.com/MEGA-MAN-X-2.html
Flame stags stage is SHORT. Although it can be hard. Especially towards the end. Going for no damage while climbing up the lava shooting ports is very difficult to master. Especially double this when going for speed.
The E or Sub-Tank can be picked up easily. You can even NEON jump to save some time ! The heart is also easy enough to get if your fast. There is NO capsule in this level.
Flame stag is probably the easiest boss to beat quickly. There are really no tricks involved at all. Maybe dashing FAST to nab in some combo's at the start of the battle.
Either way just use Bubble Splash. Dash jump opposite of Flame Stag and use bubble splash again until he is done.
One more note. Bubble Splash when charged, forms a semi - shield around Mega Man X. I suggest trying this method when going UP the volcano towards the levels end. Dodging the fire ports is hard. Very hard indeed.