I didn't even know biology, chemistry, phys were weedout classes until after I took them. I really wouldn't say that any class is a "weed-out" class, they simply ask more than students want to give. If that's what you call a "weedout" class, I'm sorry pal, but life itself will "weed" you out until you have nothing left.
As for advice you can use, take each class separately and adjust accordingly. Some of my biology classes were a joke but others were a pain in the rear. Lower/upper division doesn't matter as much as the professor you have and the level of interactions with your classmates. Study groups make a big difference.