There is one thing that makes Charmers the best class in the game and that's their Level 7 Perk Expert Inspiration which doubles duration of buffs cast by the PC and is by far the best level 7 perk in the game period.
If you are going to go charmer mage, I would definitely structure your powers to take advantage of this perk.
My charmer PC is Agility, Willpower, and Prescence focus.
My Armor is the Crown of Winter (required for a power, sorry Ryn), Izkanti Dress (Triggered armor buffs are also doubled), and Guldring. These are the armor pieces I would consider non negotiable.
My Powers are, Bardic Cadence, Blessing, Sorran Chant, Dawnbreak, and Aurora of Vigor (required to be a valkyrie).
Trust me, almost everything will be nuked by a blessing (which is two turns which is insane) followed by a dawnbreak and anything that survives is going to face insane weapon buffs. Aurora of Vigor keeps anyone from dying for 4 turns. Sorran chant that is empowered by blessing is almost a full heal and another weapon buff.