If you have the chance to get Thunderclap (lvl 5 multi encounter) from Oma, do it. Trust me.
For my standard rotation,
First round, I pop Thunderclap right out the gate,
Second round, hit the tank - or the threat with high storm resistance - with Primal Scar (lvl 6 single recharge) to weakening storm resistance,
Third round? Sit back and watch the light-show. Usually 250+ damage per enemy, and ~375+ with a critical hit. Of course, depending on how much you lower resistance, you can get in the 900s, but sadly I have yet to reach 1000 ):
Anyways, anyone who ain't fried can get finished off right quick with your remaining Recharge spell (I too like Cryostatic Arc, but Sorran Chant buffs your team's storm damage which helps for standard attacks) and with your Charm ability.
No real preference for Ultimate, but I usually go for Abjure in case I need to hold off a heavy damage dealer til round 3.
For equipment, I really like to lean into Karithea's Pyromancer's Set's 4-item bonus, where Spellpower is equalled to Sexiness, if it's higher. Perfect synergy with Charmer: now that we no longer have to worry about balancing Spellpower, strictly focus on maxing out Sexiness instead. If you're Charmer, I'm assuming you've been maxing out Presence already, but if you haven't, then you should respec and rectify that. The Stylish! perk is your best friend: While not wearing Heavy armor, your Sexiness is increased by 4 times your level. You play things right, you usually get both Spellpower and Sexiness to a minimum of 130.
A secondary thing you'll wanna bump up is your Crit Effectiveness & Crit Chance values with companion boons, max. Cunning drink buffs, and accessories such as the Hornet Ring. This will give your Thunderclap an even more devastating level of damage to at least one of the unlucky foes that's hit. Shoot for a minimum of 30 Crit Chance and 200 Crit Effectiveness.
If you have the Dawnsword, that'll be a good primary. May not be a catalyst, but the Deflection at-will stance will save your ass from DPS while you wait for the thunder to strike. Plus melee helps counter silencing, especially with Wing Of Asira as offhand for immunity to the Disarmed effect.
This is usually my go-to for Dark Mode, with a support companion and a tank companion.