Basically, the general strategy with the charge blade is to never convert energy into phials when it's glowing yellow, only red so you get the most out of each recharge. The gauge will gradually go from no glow to yellow to red as you attack in sword and shield mode. Once that hits red, hold R and press A to convert those into phials. Once you have those, the A attacks in axe mode get a serious boost to both KO ad damage, at the cost of one phial per attack. You generally want to go all out whenever you have three or more phials, and try to recharge in sword mode until you're back up to five. Also, if you realize you're going to miss with the axe mode A charge attack, press R to change the attack into a boost for your shield. That will make your shield as strong as a lance's for as long as the buff is active. Normal attacks in axe mode without phials are very wimpy compared to how much you could be dealing.
tl;dr Charge in Sword mode until red, recharge and mash A in Axe mode until phials are low, rinse and repeat.