Mashal is fun and if you get her rolling, she can be a solo in Deity and below. However, in Demonic, her bulk does not stand up to too many of the enemies' offensive capabilities. Her having access to the Ascendant class makes her a candidate for B tier.
Mashal is an odd one. She wants to be taken out of Gale as soon as possible and needs to stay ahead of the curve for her passive to actually be in effect. She finds a lot of success in Monk early on for the mixed growths and likes to be among your first promotes to keep this base class. Alternatively, you could just put her in Conduit. She will have worse growths here but she likes the 1-2 range and the crit passive.
As mentioned above, Mashal really enjoys being among your first promotes. This is to keep the Monk class for Charged Ki.
Stormkeeper - This is Mashal's best build. Why?
access to 1-2 range with a weapon that will crit and crit hard
Charged Ki giving her the stats needed to double and avoid being doubled
tier 3 passive gives her mana back from overkill. (she will be really needing this due to her passive)
Kindling Ring giving her power every time she crits with Flame Infusion active
Victorious Ring giving her really respectable sustain synergizes with tier 3 passive
ability to heal self with Ki Blast or self and allies with Lightning Strike in a pinch
tier 2 passive weakens enemies that can counter attack helping increase her not amazing but respectable bulk
with just a little setup (like an AoE ability that you were probably going to use anyway), she can easily get back all the mana from Charged Ki and Flame Infusion in overkill damage mana conversion via Obliteration passive
Ascendant - She can run this and be just as ordinary as anyone else. While an S class, unless you just want to ignore her passive, I really think it may just be better to run her as the Stormkeeper because her personal skill can actually become a detriment due to not having enough mana to use Divine Flow / Charged Ki. You can totally ignore her personal skill though and run the standard cookie cutter Ascendant build.
Godfrost - She actually isn't terrible in this class, but her low fortitude and mastery make it not synergize well with what the class wants to do most - tank.
Tempest - You'd think she'd be decent in this class as she started in Gale, but not really. Unless she gets a lot of favoritism, she'll lack the power, speed, and avoid needed for this class to perform decently.
Mashal is an interesting one. You want to be a higher level than the enemy to proc her passive. So you want to always make sure she gets a level up or two every map to stay up to date with her.
Early on, you can either keep her in her base class or make her a Conduit. When you reach 25, you'll dip into Monk for the Charged Ki skill (a little bit of survivability with Nerve Strike passive too) and then upgrade into Stormkeeper.
Stormkeeper allows her to do 3 things:
Recharge mana from her passive
Have access to 1-2 range
Have access to a decent stat spread further enhanced by Charged Ki.
You want to make sure turn 1 you activate Flame Infusion. This will proc Kindling Ring giving you 1 power and upon activating it every turn. Moon Wand will give you decent Crit as well as power that scales for every 4 Mastery you have. Pick up the Crit damage and let Victorious Ring keep you alive way longer than you'd think.
Around 20 mastery or so, the +5 and mastery scaling for Flame Infusion will be about the same. At about 25 mastery, the scaling is stronger.
You'll pick up the Charged Ki mana efficiency and cleanser.
Lightning Strike is a clutch heal that just adds to her versatility.
You won't hardly use anything else.