Red Kihunter Shaft
|Amphitheatre||Adjacent rooms||Lower Norfair Fireflea Room
Red Kihunter Shaft Save Room
The earliest you can lay the last power bomb is when the blue line in the screenshot reaches the left edge of the screen as you're moving right. Any earlier and the yellow door will not open.
At the start of the room, the fastest way down is to fire an angle-down shot as soon as you enter, then fall into the hole and downback through the hidden section.
With Plasma, you can "hitbox" through the Kihunters by hitting them with a shot as you run through them. You can also use two charge shots (or one double-hit) to kill them, which reduces lag when the Power Bomb goes off.
It is possible to clear two of the three bomb blocks at the bottom of the shaft with a Bomb Spread from the ledge just to the left. This is necessary if you used Power Bombs in the Pillar Room, because you will need one more in Wasteland to avoid picking up the "Power Bombs of Shame".