One strategy for Angry Birds Free Bad Piggies level 21-2 is to fling the Yellow bird through the rope holding the boulder and into the bouncer beyond. The bird should ricochet into the wooden support above the level, causing stones to fall onto the piggies below. As needed, use a Yellow bird to clean up any pigs that remain.
This is Level 21-6 from Bad Piggies in the full Angry Birds app. Feel free to check out that page for more strategies.
If you have a different method that is easier or yields a higher score please feel free to leave a comment below explaining your strategy. Please don’t post your score in the comment section without an accompanying explanation on how that score was achieved. For those looking to post individual scores, head on over to the leaderboards and compete with the rest of us!
Yellow Bird into the top of the boulder where it is deflected, cuts through the rope, hits the pink trampoline and flies to the left and hits the vertical wood block under the stones. All four stones fall.
Thank you @ernor63 for the strat and @eweber for the post.