I play it with [Zephyr], in the sky, far from the huge explosions. The goal? make hell on earth, without a corner for ennemies to hide. Mostly choices based on fun, but works pretty well. This riven makes firing from above nice with projectile speed, i'd recommand the [Terminal Velocity] if you don't use [Aerial Ace]. Can be used while static in 1 and chill, watching the ground ablaze. Or while jumping around between some shots and aimglides whith [Aerial Ace]. Is it the strongest build in the category? No clue and I doubt it, but it feels very strong to me and it's a pleasure to use, watching your ennemies melt in sorrow from afar.

Some mod choices :

I didn't test too rigoously but here is what i remember. Logically speaking, fire and slash should be redundant for dealing with armored units or infested and make no difference for shield as well, but it somehow seems particularly effective. [Hunter Munitions] is overall much better than [Serration] (not surprising since there's some damage in the riven). With HunMun on, [Hellfire] is overall better than toxin I think, but the difference was probably small. I could have expected toxin to be much better because it bypasses shields, but it's less effective against armour, and for units with both shields and armour it's very weak. [Vigilante Armaments] and [Serration] were very comparable in simulacrum, but I just like having more bomblets, lets me aim less and cover more ground. (All tests done against lvl 130 ennemies, various factions). Note that I use 5 vigilante mods out of 6, thanks to [Helios] + [Vigilante Pursuit] (never too much detection range when you're so high), haven't tested without. You could take [Vigilante Armaments] instead, though I never had ammo problems.
Without the riven, please do test but I'd recommand [Serration] instead of the riven, and status chance would be nice. I don't have [Hammer Shot] to test, so maybe that instead of [Vital Sense], or either [Rifle Aptitude] or [Thermite Rounds] instead of [Vigilante Armaments].

The helpful Zaw :

Although even against fat shields that slash can't harm fast enough, this [Zarr] would eventually kill anything because fire infinite stacks, it's weakness is still big shields and worse : shields + armour. I like this style of being up high and not aiming much. You could probably use some explosive secondary with long range and slap toxin on it, or some heavy [Glaive], or a sniper weapon for tougher targets, or coming down to the ground, many options. But i like the idea of exodia contagion + [Aerial Ace]. Now that we're gonna use a whole weapon to help the [Zarr] bring doom faster, what could we want? Toxin is a given, that's why we want it. Since it's possible with plague zaws, let's be greedy and get viral as well, this way slash and fire will thrive even more. We won't necessarily be using it too often and it's just supposed to help the [Zarr], so slapping as much viral status as possible seems good, and toxin status definitely won't hurt, and the [Zarr] is slow, exodia contagion projectile speed is only affected by mods, only attack speed is affected by zaw raw speed, so i've decided it's ok to go with the zaw having the most status most damage, least crit and least speed. Still 2 choices, between scythe and staff. The staff has more damage, the con is range, which we don't care about because exodia contagion doesn't care. Conclusion : i chose [Plague Keewar] + Korb + Ekwana II Ruhang.
So we got a zaw which makes the most out of every strike (good because [Aerial Ace] doesn't let us abuse it infinitely) and splashing tox + viral status. The sequence i recommand most is : bullet jump + contagion + jump + ([Zarr] + contagion + jump) x [Aerial Ace], where you can abuse the [Zarr], then use contagion whenever needed (if you aim the whole time, just melee) before jumping again to have contagion ready (and let the jump be refreshed again while you use the [Zarr]). You can get into 1 right after jumping, you can still use contagion at anytime (breaks 1, but then jump + 1 again). Might be complicated for what it gives, i don't know, but i find it comfy, fun and more than strong enough. And those grounded ennemies don't stand a chance~
My build is here, for now a work in progress https://overframe.gg/build/171269/plague-keewar/flying-contagion-test/

Hope this helps you have fun!
And do use with caution, or you'll fly as much as your victims :P