Start with Chile and Argentina. Find the country (not guessed) (number it 3) that borders both those countries. Then, find a country that borders Argentina and country #3. Then find a country #5 that borders country #3 and country #4, and continue until you guess all countries.
Technicalities
Assume all unguessed countries are candidates.
Start with the highest # country.
If any candidate(s) border that country, narrow the candidates down to those neighboring candidates.
Else, keep the candidates as is.
Repeat for the next highest # country.
Continue until one candidate is left, and guess it.
If you get through all #-countries with multiple candidates, choose the (geographically) biggest one.