Another thing is how exactly do I give my Pokemon EVs? And how long does it take?
Edit: Also, would it be a good idea to breed all my Pokemon, to start from level one with fresh stats?
Venusaur may be OU, but it only performs well in Sun teams, and only if it has Chlorophyll as its ability.
Read this; it definitely helps explain how to get EVs.
With Samurott, you'll have to make do with with the one that Prof. Juniper/Bianca gives you. Zweilous can be found in Victory Road (in the caverns that lead to N's Castle). You'll have to do some breeding with Ferrothorn, yes, since Leech Seed and Spikes are two of its egg moves (breed a female Ferroseed/Ferrothorn with a male Cacturne, as the latter can learn both moves by level up. But you can save time by breeding a female Ferroseed/Ferrothorn with any 'mon in its egg group with Leech Seed, then teaching the offspring Stealth Rock in Nacrene at the cost of 10 Green Shards). Defiant Braviary appears every Thursday in Route 4 (White 2 only). Metang (and by extension, Metagross) is so hard to catch; just breed Beldum/Metang/Megagross with Ditto.