Basically, you'll want to set a custom timer in EonTimer with the following timings:
1. ~3 sec (you'll SR at the end of this)
2. 3.35 sec (you'll hit L+A+L+A at the end of this)
If Charizard/Venusaur's cry plays any time after during the title screen music begins, don't bother checking the seed. They'll be more constant when you get the timing just right. You obviously need to set L=A, but seeing as this allows for faster button mashing, the game will not register L+A+L or A+L+A as three presses if you're doing it at an adequate speed. A+A+A (or L+L+L) is not recommended because it's simply harder to pull off with perfect timing.
Yeah, I have the save for retail cart stuff. I am going for a SD Mewtwo, and hopely get a retail cart one. I have an idea how to do this, but because I don't have Rare Candies yet, I can't test it yet. But yeah, I would really want some advice :p
You needed a LeafGreen save?
Based on what you've done before I'm assuming you want to try retail cart RNG. If you decide to go through with it I can offer some advice, but keep in mind I haven't been able to pull off a project yet.