I think 15 seconds is very littleSince it wasn't mentioned: I think the toxic wasteling size increase caps out at 3 eaten critters.
If you want to do this in a place without critters, and are very patient, the wasteling WILL eat critters spawned from the Critter Hand Cannon toy, although you can usually only get it to eat one before they all despawn. It has a 5 min CD so 15 min to get to 3 stacks.
Edit: Also, pet summons persist through mage teleport spells so once you've got a big boy Bluestacks Kodi Lucky Patcher you can tp wherever
I can confirm Magical Saucer works - you're free to mount up and fly around, it just moves in standing animation with a tiny saucer floating beneath it.
The Toxic Wasteling x3 + Pet Biscuit stacks with the Big Red Raygun buff (only 15 seconds though) if you want to reach Maximum Lad Status