About megistos and hazuki

Your team is already good enough.

Lofia and Logi are doing pretty well in arena. Same for zangetsu I think.

Megistos isn't that used honestly. In JP he's pretty low in tier list too. Not sure about hazuki.

Not sure who Megistos is but I have Hazuki, If i remember right she is placed a little low on tier lists.

Your team is already good, the only reason you'd want to add Hazuki would be if you like the challenge of using a mage or the mission enemies are immune to physical attacks (the alchemical urn event).

Running a mage can be difficult as they are pretty squishy and they usually take a few turns to charge their spells. I run Mia (4* mage) and I often find that sometimes I can use her really effectively but at other times it would just be quicker to run another type of unit. I'd definitely make sure though that you have a few mages levelled because as mentioned before there is an event where only magic works, it gives alchemy urns as rewards which aren't useful to most at the moment as they seem to be used to enhance stuff.