Well I can tell you for certain I killed it very undergeared with the charge blade and would disagree.
One thing I would say about this game is that it's not about having "the best weapon for that mob" per say, but more about knowing your weapon as well as you can so you know where your hit windows are on a mob. There are of course slightly better and worse weapons for certain mobs but I think in general the most important thing is how well you know your weapon.
I say all that because the charge blade is probably the most flexible weapon in the game. Within charge blade you have sword and shield which moves fast while drawn and hits rapidly, can charge and you use it to charge your sword/shield. You can also draw directly into axe mode which is a "large weapon type" that moves slow and has big overhead chops you can use on a mob like anjanath if you want to hit tail/head in certain situations. The most powerful attacks and what really makes charge blade OP is the charged slam axe attack which definitely leaves you open to getting hit... but it also is highly likely to stagger the enemy. Also you know... shield!
All of that sounds great but it's really more about knowing where those things fit into each mobs hit windows. If you know longsword better, especially much better it doesn't help to switch weapons at all even if the kill time is half because it's all about efficiency.