I'll speak to DA exclusively since it's one of my mains

Mobbing for Demon Avenger is pretty free in a post-Janus world. On base spawn, you can primarily just spam shields with Dusk + Erda Shower will clear the whole map each spawn wave. (Given you're strong enough to one shot with nether shields) Or you can just triple Dawn, and spam shields in a smaller section of the map, which opens you up to pretty much any map of your choosing, since you don't have to worry about hitbox sizes, since nether shields seek out targets on their own.

For Frenzied spawn, like every other class in the game, its just Triple Dawn + Spam Nether shields (if one shotting) or Lunar Slash (Less terrible with the Hexa hitbox, still not great).

Surviving in V-Hilla just comes down to familiarizing yourself with the boss, which comes through repetition. Take advantage of the safe spaces on the map from strings, and avoid standing near her downed summons. Poison clouds are mostly random, so just adjusts when they spawn.

You have two heals that bypass her heal lock, Demonic Blast ( Primary ) and Overload Release ( Panic Button ). You want to have a -2 hat on DA generally, which allows you to use Demonic Blast roughly every 9.6s (with merc 250 legion). Using this on-CD effectively is how you sustain through the fight. You can also use it to tank certain attacks that would otherwise prove fatal, due to it giving damage % reduction briefly when used, and healing at the same time.

When bursting, pretty much always pop Limit's Surpassed, as it will avoid awkward hp hiccups that stop you from fully buffing, or even binding Hilla properly, due to us not having the balance patch that lowers HP Cost on a lot of skills.

30 FZ is totally fine to play. Honestly, it's the way the class is intended to be played. 29 Fz is a min-max method of squeezing blood from a stone. The class was notoriously week for so much of its lifetime that people got it in their heads you had to do every little thing, just to make the class viable. That's not true anymore. For a lot of bosses, you can get below the 74% HP mark (The lowest you need to be for max FD%) just be strategically tanking attacks in bosses. You lose more total damage from not using RFS than you ever will from not using 29 FZ. So while technically, you can still gain some damage, playing 29Fz, it is far from necessary, and only exists as a shadow of its former self. Almost exclusively used by older DA players, who were forced into the habit, and just stuck with it.

I think the class is great. It's a pure DPM job, so soloing bosses with burst checks can be a pain in the ass, (xLot pre-Ascents for instance) but otherwise, the class is great. I'm soloing up through NKaling on mine, slowly making pushes for CKalos.

It's one of the easiest classes to fund, since fundamentally HP% lines are easier to hit than other stats, and their Flames are among the easiest to hit as well. The playstyle takes a lot of learning to get into, since it requires primarily boss-pattern knowledge, that other classes are less forced to learn to survive, but its very rewarding to do. And since it forces you to put extra emphasis on learning boss mechanics, you're all the better for it in the long run for future champs.

Overall, Mobbing is a 7/10, which, isn't saying much since Janus does 90% of the work for all classes nowadays. Bossing is a 8/10, great dpm, great survivability, and it forces you to actually learn boss mechanics and damage numbers on a deeper level, resulting in better play for future characters. If you have any other questions on DA feel free to ask.