252 SpA Ninetales Fire Blast vs. 252 HP / 0 SpD Assault Vest Armaldo in Sun: 160-189 (45.1 - 53.3%) -- 33.2% chance to 2HKO
Armaldo isn't a particularly good counter to these Pokemon; it has poor typing in that it doesn't have a lot of resistances and only base 80 sp. def. As a switch in, it will still have to take around 50% damage from Fire Blast before being able to do anything, not to mention it also has a Stealth Rock weakness. Aqua Jet will do jack in sun, especially with no STAB.
The best counters to these fire Pokemon is Tyrantitar, especially when packing Sand Stream & Assault Vest. If you don't want sand stream, then Assault Vest Goodra is a viable switch in. Scarf Terrakion will OHKO most of them with a Rock Slide, but cannot be a safe switch in.