you´d have to post a screen shot for some help with this particular fight. But yes, the second wave can have different unit types from the first wave (if the first one does not have 3 different unit types), that can kill your mages. If for example there is Light Melee units in the second wave, you may be better off using 5 Heavy Melee units. They will not do as well in the first wave against Heavy Melee, but much better in wave 2. If the second wave has Light Range Units you may be better off with 5 Heavy Range units. Or potentially a combination of units. All depends on what and how much you encounter in both waves.
Sounds like fighting isn´t usually your thing? Some general ideas: Helper buildings, Feed your Fire Phoenix, upgrade military AWs, use manual fight, ...