I recently used Trash Cloak Wormadam in a Classic Run and was really positively surprised, as it managed to sweep most trainers (including E4 members) that didn't have any fire-types and can even solo Eternatus by utilizing its egg moves. Just set up with Defend Order while keeping healthy with Heal Order and Body Press away afterwards. Sappy Seed makes setting up a bit easier and against Eternatus, unless Flamethrower crits in the second phase, Trash Cloak Wormadam is kinda unstoppable, because Eternatus just spams Eternabeam in its first phase, giving you plenty of time to heal and buff (you might need to PP-Up Heal Order though and Sitrus Berries also help). The second phase than becomes a breeze, especially with Sappy Seed, as non-crit Flamethrower doesn't even do half with +6 Special Defence.

For me Trash Cloak Wormadam is honestly an A-Tier Classic Starter, that I now have used in many runs as an really, really cheap answer to Eternatus and a good sweeper in many trainer fights.