First, let’s take a look at its stats. It has a not too bad BST of 489. It mostly serves as a hazard settler because of its amazing bulk with 131 Defense and 116 S. Def, it is likely to take a hit. Compared to Galvantula’s 60 Defenses, Ferrothorn is much more likes to take a hit. However, Ferrothorn does have a 4x weakness to Fire, a common type to use in competitive. Although Ferrothorn’s Speed is much lower than Galvantula’s they’re offensive stats are about the same. Ferrothorn also has access to Thunder Wave so no need to use Galvantula’s Thunder Wave. You can also give it a Focus Sash to withstand at least one hit.
Now, let’s Look at Galvantula’s stats. It has a lower BST of 472, but still good to use. It is substantially faster than Ferrothorn and has a bit higher of an offensive stat. You may use this as your hazard settler but I recommend Ferrothorn instead because of its higher defensive stats. Slowing down the opponent is good, though. Nevertheless, I still recommend Ferrothorn because Stealth Rocks and Spikes both deal damage to the opponent, resulting in a higher damage output.
Hope I helped!