Can I defeat Spiked Beetles without a shield?
They can be flipped using either the shovel (with some timing) or the gale seeds also work
Additionally, (though not quite on-topic), you can use the like-like ring which can be obtained in Gasha seeds, mini-games or Maple meetings, to transform into a like-like, thus becoming impervious to being eaten and having your items stolen.
Ben is right about what you can use to defeat the spiked beetles.
You can also use a Gale Seed to go back to the dungeon once you've been there (there's a seed tree right below it). No need to go through the forest again.