What's the optimal strategy for the first Pokémon battle?
Every main Pokémon game has you battle your rival or friend after you receive your starter Pokémon. Every starter begins at level 5. They each know one Normal-type physical attack (e.g. Tackle, Scratch) and one stat-altering move (e.g. Growl, Leer), aside from Pokémon Yellow where Pikachu gets Growl and Thunderbolt (with STAB; the rival's Eevee also gets STAB on his Tackle.)
Throughout the Pokémon series, what's the optimum way to deploy your two moves in the starter battle?
I had a play with a damage calculator I found online and my conclusion is: there are way too many variables to produce a definitive answer: base stats, IVs and Nature all play a part, even at level 5.
If your Pokemon has perfect IVs and beneficial Nature and your opponent has 0 IVs and negative Nature, it looks like you could deal at least an extra 20% damage than if those stats were reversed. I tried a good Snivy against a bad Tepig and got 42% damage from using Tackle, whereas a bad Oshawott against a good Snivy only did 14% damage with Tackle.
In my experience, using any status move is not worthwhile. It always takes the same number of moves before your opponent is KO'd. Example: assume an average damage of 20%, that would normally take 5 Tackles to KO your opponent. If you used Leer first, Tackle would deal ~30% damage (20*1.5). Now it takes Leer + 4 Tackles, so you don't gain anything.
Also note that your opponent uses moves randomly, which you can benefit from: Tackle>Leer>Tackle always does less total damage than Leer>Tackle>Tackle.
My advice: never use more than one status move, and if you do use one always use it as the first move. Then just spam Tackle.