Got some new mons, could use some rune recommendations..

Max damage per hit is always rage/blade with around 70% crit (and buffed to 100 so you have more crit dmg subs) assuming 6 stars with good substats. Before that it's whatever you have best of and broken 6 stars in 2/4/6 if necessary since they're so important. Amazing subs on fatal, can't build rage where you have good crit rate and crit dmg together? No reason to, then, go fatal. But eventually rage is plain better without special factors like built in attack or crit mods, just how the math works. Rage is also much more likely on this guy when he's selfbuffing crit on his 1st, and that buff does not need crit to activate like a Sylph. You can totally run with 70 and always proc it to cap yourself without assistance. You would, however, need to use skill 3, then skill 1 on the free turn, then wait a turn to hit with Drive.

The single target attack/spd debuff hits 100% via skill-ups, which is good, and the 3rd skill granting another turn like other good buffs (like Fire Chimera) is good, too.

That being said, what would you plan on using him for? I don't see any real reason I'd bring him to any PVE content, which leaves PVP. The most I could see is some group that was trying to not use Shannon or Bernard or something in GB10, and this guy would cover attack speed debuff and attack. Depending on comp, the def break may not affect kill sped that much, or is doubled by Bella. However, you'd be missing glancing and def buff without Shannon. If you tried to just avoid Bernard, you don't have his spd / atb buffing. And if they were in the group, I don't see any reason for this guy to be there, there would be a different DPS I would take. No AE without huge boss-killing potential like Orochi, which doesn't persuade me.

Why do you say potential for a high damage attacker? That is entirely dependent on his multipliers. People tested, skill 1 is a standard multiplier (aside from the dot on crit for water), Drive is being estimated at 498% on a list on reddit (can't verify whether the guy tested it properly), and for reference, a hellhound's double attack is 330% x 2. Double attack is quite a lot of damage, so this is more average-ish To use a wind creature, the wind grim reaper, who's considered the worst reaper by far, has that finisher that hits for 268%x3, and no one uses him, so 498% in a vacuum is not very bursty. You'd be compensating for the damage with utility, except with the mons that exist in wind, I can't see when that would get used. The lack of harder hits, and an anti-wind pvp meta, kinda kills him there.

All the self buffs are cute, the fact that he's so self-sufficient, I just don't think it's enough. He's probably my least favorite of the new sharks. People went the most nuts for water of the base 3, can be built as a soloer while going for raw damage because of the heal on kill (or vamp to sustain between killshots in exchange for losing some topend), and the crit on hit made someone use one in a DB10 team. DB10 is surprisingly flexible once you have good runes from doing DB10, so it isn't a miracle "this is how to have an easier time breaking in" like the recommendations for light trio and shannon/bernard to get people a foothold in pve. Still, it has an interesting kit because of that passive. Passives on 3 stars tend to make more interesting/stronger mons.

The fire mermaid people got excited about. Heal with a swap to rez situationally with the CD shifting can be nice, it's fire, which is nice for the meta, and stopping skills with cooltime can be a very strong ability, even single target. Obviously this is a lot stronger manually, as you have to aim it, which the AI can't properly do. Zaiross with speed and a tank is scary on D because he might be runed above easier gank range, and if you go in squishy enough to try to gank and it fails, you probably collapse when he breathes, which hits everyone. It would hurt people using Chasun as their wind tank, though, instead of a wall like Ermeda that didn't care. Your AI would go right after her (or Ramagos, who's another popular one for pivvip). First skill scales on hp, which is nice considering she'd be built as a support anyway. I haven't seen a good calc on the mermaid attack 1 yet, but someone said Wind seems to be 200x3 on its 3rd, and that has debuffs baked in too, so I could expect 500-600% on the fire one. No scaling off hp or anything though, and you likely aren't building it as an attacker.

Most likely Violent/xxx is the ideal, like anyone with important skills you want to get back ASAP. Before that, likely swift/xxx, swift/focus unless you have enough acc from subs, and even then I'd rather do /focus and use subs to cap so I could do spd/hp/hp on a support like this. I don't like the 50% chance of dispelling RNG on her skill 1, but it's a skill 1 and a strong effect, can deal with that being RNG. You don't want to miss her 3rd, ever, though. I mean besides the stuff you can't avoid. Definitely needs acc cap. Not sure yet where she'd fit in a DB10 or certain ToA stages, but should be quite a strong part of a toolkit for PVP. I pulled the wind one from a drop scroll, and I'd totally trade it for fire. (Or dark, oh my god, do not look at dark, Betta is *INSANE*)