It's an uphill battle since the premed courses are designed to weed students out, so studying a bit harder is not really a viable option. Also, physics has all of nothing to do with biochemistry (there is 0 overlap).
1. Upper-level physics is very very difficult to do well. So saying blindly that you will get an A+ in upper level physics is terribly misinformed. And besides, physics is harder than any biology course by a longshot, and that's undeniable.
2. No one cares about premed courses being a weed out. They are required for a reason, and it's expected you do well on them. Take biochemistry, which is strongly recommended/required by many medical schools, and aim to get an A.
Edited accordingly:
If it is difficult to accept that physics classes (esp. medical physics) are easier for some than bio/orgo/biochem classes (it could be argued that they involve different skill sets, but that discussion belongs in another thread...), then consider the question in the OP to be purely hypothetical. Thanks.
1. I'm sorry, but you don't know what upper level physics entails. And that's fine. Upper level physics isn't like "medical physics" (whatever that is). Biophysics isn't "easy" as you think. To keep it very brief, physics requires a combination of excellent math skills (not precalculus; I'm talking linear algebra, differential equations (ordinary and partial), abstract algebra etc.). It is time consuming. You will mess up on exams and homework if you make a slight mistake halfway. It's a torture and it will kill your GPA unless you are extremely dedicated and aren't too busy with ECs.
2. Saying physics has different skill sets from chemistry is nonsense, considering that chemistry is built on physical concepts (think physical chemistry, analytical chemistry, and inorganic chemistry). From which, advanced biochemical principles (mass action kinetics deriving models more complex than Michaelis-Menten) can be derived, resulting in computational biology. Biology and medicine viewed from a physical standpoint is incredibly significant, so no, biology and physics aren't different. They are approached from similar direction (understanding concepts; not memorizing aimlessly).
I am sorry but I'm being optimistic by assuming your concerns are naivete. It is a well known fact that physics is the hardest undergraduate major, possibly rivaled by a math major with a pure math focus. This isn't elementary stuff. It's hardcore, time-consuming, and mind-wrecking material where you must be desensitized in performing suboptimally.
You can do physics if you want, and no one cares. Just don't blindly expect an A+ (or even an A-) out of the blue or claim that physics is easy, when it really is not. I already answered your biochemistry question previously.