Increasing level works by interacting/feeding berries or by using
rare candies. It's not really linear, the amount of EXP is
randomized in the range and you give more exp the higher level the
Pokemon is, but higher level Pokemon still get harder and harder to
level up every level. Yes, always bring a higher level Pokemon in a
fight. Over level 100 Pokemon still have the same stats as level
100 Pokemon, but a level 100 Pokemon with terrible IVs still has
better stats than a level 25 Pokemon with 31 IVs most of the time.
People often think the PH sprites are bland. Do they shade? Well,
yes they do. You'll be surprised how much care is taken for these
sprites.