Inject a solvent on the three plants (outside the ICU, rooftop and room 203. Then, go
inside the ICU and kill the huge plant.

Not sure what do you mean by good ending, the possible ending for Flashback as follows:
1. Killed huge plant, defeated Axeman and escaped.
2. Killed huge plant, did not defeat Axeman and escaped.
3. Killed huge plant, and don't escape.
4. Help Reagan find her daughter (head back to the cabin before the bridge collapsed), and
escaped through the "Big suspension bridge" (only on Hard mode).
5. Don't help Reagan and escaped through the Big suspension bridge.
6. You die.

How do you get the 'Illusion' ending?
Do you complete the level if you wait on the other side of the bridge where you came from?
How do you get Allysa's alternate super secret ending?