Will the Snow Venom be good enough for Diablos?

EDIT: Doesn't matter anyway, I killed him with the Blango Decimater (and a crap ton of bombs).

Because I need to face him to unlock the Los and Ian urgent...

Well it will work with enough skill but I reccomend getting a better dual blade.

Grash...I think thers a topic for these kind of questions,anyway snow venom is good emough IMO. But I would try to further upgrade,
Also I never understood having to do blos to unlock stuff...but if your legit stuck you can use the ledges and gs charge him to death,tis how I did it originally

It might do, but I don't really reccomend duals against a Blos, mostly due to short reach. I'd reccomend a Greatsword or a Longsword, or even a Heavy Bowgun.

In the case of Greatsword, do what Laoshanicrus said: Get him in the walls (back to the wall, guard against the charge), and unleash hell on the Blos. Don't aim for the head, as it'll render this tactic useless when the horn breaks, but instead for the tail. Apart from that being his weak spot, you'll also get a free extra carve.

With a Longsword, try to stay under and lightly in front of it. Spam attacks the legs, hit the tail when there's an opening. If you're gonna use the walls, the only variation is having to dive out of the way.

With a Heavy Bowgun, the main feature here apart from, yes, ledges, will be shields. If I'm correct, you should have the Daora's Delphinidae available. It can freely use Ice and Water S, all levels of Pierce, Normal and Pellet, and to top it off Crag and Clust S Lv1, and has natural Weak recoil to boot.
With or without the Delphinidae, thanks to the shield you should be able to defend yourself from most of Diablos' attacks without losing too much health. If you're not sure about being able to evade the attack, block. I suggest getting armour with the Pierce S Up ability.
Thanks to this last tactic I did my Nekoht 8-star Urgent...using Low Rank gear! (specifically, Remobra with Cenataur Coat).

I'd suggest a longsword. The Blango longswords really worked well on my first try on Diablos in Unite! /yes/

^
I did tha quest today and that's the weapon I used. Upgrading it's easy enough if you get the tails out of the way, just do the dual blangonga quest and capture both, 2-3 runs of that should get you all you need.