Congratulations! With the very small number of transfer students accepted, you must be a very strong student and should enter Williams feeling proud and confident that you can handle the work there.
When you go to a top college like Williams, you have to keep in mind that everyone there was a top student in high school and everyone there is smart. You might not get straight A’s anymore, and that’s okay. Don’t worry. Grad schools know Williams is Williams.
But I don’t think you are at risk for failure or for many C’s, barring personal emotional or study habit difficulties. It seems safe to say that grades of C or lower seem relatively rare.
Yes, you can get an A with hard work. There is not really grade deflation, just a high level of challenge.
Read the course reviews. Some professors grade harder than others. Sometimes you might choose to take a course where the professor is known for hardly ever giving A’s if the professor is also known to provide a wonderful, intellectually challenging, deep learning experience in the class. Being intellectually stimulated and having great class discussions is arguably a more important part of the college experience than getting straight A’s.
Use office hours and services like peer tutoring in math or in writing. Help is there if you need it.
If you do not already have good study habits, start working on them. Most of your time is spent on your own, not in class, so you will have to set a good pace for reading and for writing your papers and doing your problem sets or labs. You might try to get your work done on the early end, so that you can bring drafts to office hours for feedback.
The few students who seem to really struggle and end up leaving (at least temporarily- Williams tries to hold onto kids and help them graduate, even if it is a few years later than their entering class), seem to be those who either have psychological difficulties or have really bad study habits because everything always came easily to them before Williams and they never learned to bite the bullet and do their work. And there is help at the school for these students, too. Never be afraid to seek either academic support or psychological counseling. You will be attending a school with deep pockets used on good resources for the students, including small class sizes and accessible professors who value teaching and want to be there for their students. Take advantage of all that. It exists for YOU.
One final tip: balance your classes. As a STEM major this may be less relevant for you, but generally you won’t want four courses with super heavy reading and writing loads in the same semester. As you look in the catalog at the incredible number of fascinating courses you want to take, try to time them so that each semester you have a balanced load.