Alto's Adventure

This will likely be one of the last if not the last achievement you unlock, especially if you farm idle time for the "Silver Surfer" achievement. Because of this fact, I highly recommend upgrading the Llama horn immediately and picking them up as often as possible while playing through each of the missions. To put it in perspective, when I finished the last mission to "beat the game" I had only collected around 9,000 llamas. I then had to spend dozens and dozens of hours mindlessly grinding llamas to unlock this achievement.

There are two ways to grind llamas in general:

1. Play the game like normal, picking up every Llama horn you find and saving every llama you come across
2. Replay the start of a run over and over and over again to collect the first 4 llamas that always spawn right at the beginning

I believe that in theory and on average, method 2 will get you more llamas in a set time ONLY IF you are not getting anymore advertisements. Otherwise, the ads every 4-5 runs will slow down your collection rate considerably.

That being said, I personally prefer method 1 as long as you have a fully upgraded llama horn. With the upgraded horn, it summons 20-25 llamas each time. It at least keeps it a little fun and interesting to actually play the game, whereas method 2 is absolutely mind numbingly boring.

The rough calculations in my experience are this:

Method 1 - with a fully upgraded llama horn I was collecting around 100 llamas every 4-5 minutes assuming I didn't miss a horn nor a group of llamas called by a horn. This means around 45 minutes per 1,000 llamas, and 4500 minutes for the full 100,000. That's 75 hours. Keep in mind though that missing any llama horns or groups will set you back quite a bit.

Method 2 - I was finishing each collection of 4 llamas in only a few seconds, but since you're only getting 4 llamas at once, it was taking me around 4 minutes per 100 llamas. This is why I said that this method is probably better on average than method one, BUT that rate of saving llamas is with NO ads popping up so bear that in mind.

Use whichever method you prefer, or a combination of the two if you'd like. Just be prepared for an insanely long grind if you don't focus on llama saving until after you complete all missions.