I have a magic system in which mages can enhance existing power sources (such as taking a small spark and building it into a fireball) or launch pure 'magical' attacks, generally their magic is only good at direct immediate applications of force/energy, no long term effects or subtler magics; but they can be quite powerful particularly in battle.
All mages store power in a Crystal which they use as a reserve to fuel their power. It can take anywhere from days to weeks to fully 'recharge' a crystal, depending on skill of mage, quality of crystal, and abundance of ambient mana. When they run out of mana they can't do anything and are helpless, and in fact generally try to keep at least half their mana in reserve at all times for emergencies.
However, an oddity of the system is that it's 'cheaper', in regards of magic used, to deflect or even redirect an enemies attacks back at them then it is to produce your own. This is because an attack requires two steps, building up enough force for the attack to be dangerous and then directing the attack. While a defender need only expend enough power to 'capture' the attacking magic to redirect it, a much lower expenditure of mana. He also requires the skill to capture and control the spell fast enough to keep it from striking him, if not skilled enough he may be roasted, but most mages can redirect a single attack pretty easily.
Non-mages can also Channel, which allows them to flow mana through their body to enhance speed/reflexes/etc to make them into stronger and faster fighters, thus solders are generally faster and more lethal then soldiers of today, with a larger difference between master solder and ameratur. Mages can not do this, the energy stored in their Crystal prevents Channeling. Thus they are no match in a purely physical fight. However, they still generally consider non-mages a minor threat since they can unleash death from range and generally kill solders, even skilled Channelers, before they can close in to do any harm.
I'm trying to get a feel for what magical duels are likely to look like in this setting. Mages often fight other mages and so will have learned how to fight. They can go up against mages that are more or less skilled then them, and may go up against mages that have more or less mana 'stored' then them (a less skilled mage may have more mana then more skilled mage at time of a duel if the skilled mage had recently expended some of his for instance). A mage has an idea, but may not always be certain, about how much mana another mage has stored when a battle starts.
What I want is for magical battles to still be interesting, trading of attacks and defense, counter attacks & redirects, that can take a little while to be decided. However, the fact that defending is more mana efficient seems likely to push me to one extreme or another, which is something I want to avoid as being less interesting.
I want to avoid is situations where mages rarely attack because it's so much easier to defend, but I also want to avoid the opposite extreme where a mage will unleash a flurry of attacks to overwhelm the enemy attack and end the battle in the first second. How can I encourage dynamic fights, preferably including different strategies being preferred depending on your skill vs enemy, available mana each side has, and even enemy preferences?