Finally completed this last night... i couldn't get past Tennant for ages!
if anybody wants any hints - (I'm left handed so) I try and keep the highest doctor in the bottom right corner, then aim to get the same one above him and drop down.
The trick is to always have 3 above him as if you swipe up, hartnell can appear below your highest Doctor and then you're screwed.
oh and its pretty easy to get to 5th doctor without even looking - just swipe right down right down right down - if nothing moves, up once, then right down etc.
It only really gets tricky after McGann