Spineblade

slather honey on the big trees, wait around for a few minutes, then a pinsir and heracross will start to fight over the honey. Defeat the pinsir and the heracross will destroy a nearby web. Defeat the Heracross and the Pinsir will destroy a nearby log.

Place Honey on a tree. Wait a bit in-game, and a wild Pinsir and Heracross should appear. Once they appear, fight the Pinsir. After you defeat it, the Heracross should thank you by destryoing the Web. If you defeated the Heracross instead, the Pinsir will thank you by destroying the log nearby.

There are some webs that cannot be destroyed however, which is involved in a different event.

Note: I believe it was changed so that the mons appear after a set amount of steps