Hi @CAlexander,
First of all, good job on challenging yourself.
I skimmed your code. Where do you want to call LoseALife? From what I see, it was supposed to get called in the LoseCollider instance, which makes sense. I assume that you got a NullReferenceException error message before you commented the code out. NullReferenceException means that a reference (“link”) to an instance is missing. You declared a variable of type GameSession at the top of your code, which is fine, but the value is null by default. I cannot see where an object gets assigned to the variable.
The second issue, apart from the code which is commented out and cannot run, is the spelling of the method. C# is case-sensitive.
Last but not least, add a Debug.Log to the loseALife/LoseALife method in the GameSession class so you can see whether the method gets called during runtime.
All in all, you are on the right track. The solution is almost complete. 
See also:
- Forum User Guides : How to mark a topic as solved