You don't even need thunder punch (on SD sets) to handle Toxapex, +2 Knock Off does 70% minimum and even if Mawile gets burned or hazed, itemless knock off is enough to remove the rest of Toxapex's HP. This is even if Toxapex is fully invested in def.
I feel like Magearna will absolutely be way more splashable due to its bulk, access to Volt Switch, and not being a mega, but in terms of sheer breaking potential it doesn't really compare to Mawile, which, as noted above, only needs neutral coverage to get past something that would otherwise be a check. I feel like the changes in SM will make it go from uber to really freaking good, not anything less.
I feel like Magearna will absolutely be way more splashable due to its bulk, access to Volt Switch, and not being a mega, but in terms of sheer breaking potential it doesn't really compare to Mawile, which, as noted above, only needs neutral coverage to get past something that would otherwise be a check. I feel like the changes in SM will make it go from uber to really freaking good, not anything less.