That would be very pretentious to give people some advices on how to make a game with C2.
This is the very first time I'm using it, I'm afraid to share a lot of wrong advices.
A few things:
Playing with the scale quality was very effective.
Making separate eventsheets for every aspect of the gameplay (IA, HUD, Bosses, Visual Effects, Music, etc...) + one eventsheet by level makes the code easy to modify.
And "trigger once" is a life saver.
...And I guess that's it, I'm sorry.
I've played enough with C2 now to be aware I've done MANY things the wrong way, and should I start over, I'd pay way more attention to the very good guides published by Ashley on the website about optimisation etc... : )