I have several friends who studied in Canada for high school and ugrad and are now in TW med schools
There are 3 ways to get in:
(1) Take the national exams with all the high school students, which is next to impossible if you did not do high school in TW.
Apply as a foreign resident.
The key is your foreigner status. I am not sure about the official name for it. Basically you have to have foreign resident status, and that means living in a foreign country ie US for the past 8 years without living in Taiwan for more than 4 months in any of the 8 years.
If you have foreign status then you can:
(2) Apply straight to medical schools through TW govt agencies in your country (ie Taipei Cultural Office or TW Embassy) with your high school grades. I believe that you have averages of 90% or higher then you should have no problem getting in.
If you don't have the grade in high school there is another option
(3) Go to a special transition school in TW for one year, and use the grade during the year to apply. I don't have much info. on this, though a friend got into med school through this route.