your overall point is good, stellar tera could see some use on breakers, but i'm not really following this sentence in particular. why is it preferable to keep your rock typing in these scenarios? you're still weak to dark and ghost, with an additional weakness to steel that makes you worse into ghold. am i missing something? because i feel like i read this wrong
Sorry, I worded that wrongly. It was more of a concept, but I was specifically talking about how with the 2x rock boost with stellar, that can be enough to break through the dark types that are immune to your psychic moves. Since boulder doesn't hit super hard, it's better as a cleaner, every little boost you can get is great. In some scenarios, yes, losing your rock typing is better. But you can get good damage against gholdengo or gambit. It's less so about the defensive typing, and more about the offensive power.
+2 252 Atk Iron Boulder Mighty Cleave vs. 0 HP / 0 Def Gholdengo: 153-180 (48.5 - 57.1%) -- 94.1% chance to 2HKO
+2 252 Atk Tera Stellar Iron Boulder Mighty Cleave vs. 0 HP / 0 Def Gholdengo: 204-241 (64.7 - 76.5%) -- guaranteed 2HKO
But what about great tusk? It can still take your hits well.
+2 252 Atk Iron Boulder Zen Headbutt vs. 252 HP / 252+ Def Great Tusk: 294-348 (67.7 - 80.1%) -- guaranteed 2HKO after Leftovers recovery
+2 252 Atk Tera Stellar Iron Boulder Zen Headbutt vs. 252 HP / 252+ Def Great Tusk: 392-464 (90.3 - 106.9%) -- 43.8% chance to OHKO
Basically, you get the best of both worlds, you can ko important targets that are most likely the defensive backbones of your opponents team.