To have the strongest Pokemon only have up to 3 Pokemon in you're team since you have very little Pokemon each Pokemon will get more expeirence and will gain levels ALOT faster. I only have 2 Pokemon and I beat the game and they both are on level 64.
Successfully collect all Pokemon in either Pokemon Ruby or Pokemon Sapphire to unlock an alternate border in adventure mode. Swablu with False Swipe Egg: Store 1 Pokemon. Zigzagoon with Extreme Speed Egg: Store 60 Pokemon. Skitty with Pay Day Egg: Store 500 Pokemon. Pichu with Surf Egg: Store 1499 Pokemon.