Well there are 18 (17?) types, including fire, grass (lol wood), water, dark, psychic etc, and they all have strengths and weaknesses, you just have to figure out what's good against what, and make a balanced team.
Although Pokemon Black and White are harder, the wild Pokemon are much higher level than other games. I was walking along Route 3 in these dark patches of grass, and I encountered 2 Pokemon (2 on 2) and they were both Lv.16 and almost wiped out my team because they were so strong. That was on ROUTE 3!
anyway, tl;dr you gotta know what's good against what, and if you want to catch higher leveled wild Pokemon, get Black/White.