Paper submission: Major revision impossible to pull off
You have a number of options. Only you can decide which is the best in your case.

(1) You can withdraw the paper from the original journal and submit it elsewhere, in the hope that a different set of reviewers will be more enthusiastic or realistic.  

(2) You can re-submit to the original journal and simply decline to do the reviewer's request.  This is perfectly reasonable, and editors will simply use their judgement as to whether the paper is still acceptable even without this revision.  If you do follow this route, be sure to make as many of the requested revisions as you possibly can, to show that you aren't simply being stubborn, and also explain professionally (clearly but politely) why you didn't do the other revision (it isn't practical) and why it isn't necessary (you already have strong statistical significance).  

(3) You can request an extension from the editor.  The 2-month revision time is almost always negotiable, if you have a reasonable argument. I recently requested an extension from 2 months to 6 because a co-author had had a baby, and the editor was perfectly happy with that.  (I offered to include baby pictures, but even that was not necessary.)

My own preference would probably be, in order, (2), (1), and (3), but you will have to decide on your own.

Edit to add that, as @jakebeal says, you could just ask the editor if they feel the major revision is necessary.  I would be a little hesitant about that, because I think (without evidence) that editors would be inclined to say that everything the reviewers asked is required, and then you're stuck with it, while if you just send in your revision as in (2), the editor might see it differently.  In other words, it's often better to ask forgiveness rather than permission.