What is the easiest way to spare Mettaton EX?

While it is not required to stay on the Pacifist path, the fastest way to defeat Mettaton EX would be to do so without blowing off his arms and legs, which means you will need to reach a Show rating of 12000 points, as explained in the Mettaton battle wikia page:

To defeat Mettaton EX without killing him, one must survive until his arms and legs are blown off and achieve a show rating of 10,000 or more; if his limbs are not blown off, a show rating of 12,000 or more will end the battle.

The page also lists ways to increase your rating. Here are some easy ways to do it:

Eating food sold by Burgerpants will give 300-500 rating points, but the Steak in the Shape of Mettaton's Face will give 700 points instead.

Early in the battle, Mettaton will ask the protagonist to write an essay about what they like most about him.

• Writing "LEGS" earns 350 points, which is the highest amount, being the 'correct answer.'

Equipping a different piece of armor will give 1,500 points as long that piece has not been worn previously during the fight.

Using the Stick will cause it to be thrown at Mettaton. He will catch it in his mouth and ratings will boost by 700 points. Repeating this action will give 1 point. Using the stick after Mettaton has lost his arms and legs will instead give 1,400 points.

Thus, the absolute fastest way to get to 12000 Rating is to give the correct answer ("LEGS") and to constantly switch your equipped armor (keep in mind that they have to be pieces of armor that you have not already equipped during the battle).

In addition, having low health seems to be a fast way to boost your Rating, though if you have difficulty surviving this fight, you may want to restrict yourself to the previously mentioned methods.

Getting hit gives a violence boost of 10 to 50 points.

Using the Pose action will give 100-1000 points, inversely proportional to the amount of health the protagonist has left.

In addition to the answer up there, if you're very good at avoiding the attacks, there is the "Boast" option in the Act menu which steadily increases the ratings during Mettaton's turn

But beware, if you get hit, you lose a lot of ratings