The new [Catabolyst] Augment [Critical Mutation] gives +300% crit chance and +300% crit damage after just 10 kills. Each reload you want to hit 3 or more enemies with the explosion to maintain the 300% buff.

The primary fire beam is best used with external flat crit chance buffs such as Arcane Avenger, [Harrow] Covenant, [Sevagoth] [Shadow Haze], [Citrine] or [Adarza Kavat]. This is because the beam only has 11% base crit and [Critical Mutation] only brings this to 44%. It does have an extremely high crit multiplier of 2.9, which becomes 11.6 with [Critical Mutation] at max stacks.

You'll notice that the reload Grenade deals red crits. This is because [Critical Mutation] current double dips giving a final CC of 496% cc (up to Tier 5 red crits). If you want to focus on meme grenade spam, use this build: https://overframe.gg/build/567784/

Build Breakdown
[Galvanized Shot]: Mandatory +120% dmg per unique status, i.e. +360% dmg with heat, corrosive, viral, which is a 62% dps increase with Cascadia Flare, with higher potential if more statuses are applied eg via external priming source.

[Galvanized Diffusion]: Mandatory +230% multishot at max stacks. Double dips status damage on beam weapons

[Critical Mutation]: Mandatory augment that gives +300% cc and cd at max stacks.

[Lethal Torrent]: Mandatory for fire rate and additional multishot which double dips status damage. If you run external fire rate eg [Wisp]'s Haste mote, [Harrow]'s Penance, Arcane Velocity or [Reinforced Bond], you could swap this for more crit damage (e.g [Primed Target Cracker]) or crit chance (e.g. [Primed Pistol Gambit]).

[Pistol Pestilence] + [Frostbite]: Gives both status chance and viral status which boosts all health damage by 4.25x at 10 stacks.

[Primed Heated Charge]: Boosts raw DPS and heat status DPS, which is one of your main sources of damage. Heat procs have infinite scaling because they do not have a limit on most enemies.

Exilus: Either [Ruinous Extension] to [Extend] beam length or Primed Ammo Mutation if you have ammo issues.

Riven
CD, MS, FR, tox are your wanted positives. CC and dmg are not great on [Catabolyst] since CC is covered by the augment and the weapon has very low base cc, and flare +galv shot cover alot of damage already, but these are viable alternatives.
If you roll MS/FR, replace [Lethal Torrent].
If you roll tox, replace [Pistol Pestilence]

Unfortunately you'll need an entirely different riven for Grenade only build since MS doesnt apply. You'll probably want CD, CC, dmg.

ARCANE

Cascadia Flare: Easiest to maintain +480% dmg, you lose all stacks if it expires but you can build it back up to max in seconds

Secondary Merciless: Gives +360% dmg but 1 stack which decays every 3s. Also gives bonus reload which is nice as catabolyst reloads VERY often.

Secondary Encumber: Theoretically gives 13 unique status effects after 3-4 seconds, meaning you get +1560% dmg from [Galvanized Shot] (1080 on top of the innate statuses), but only after a ramp up. This might be a good option in endurance where enemies take longer than 3 seconds to kill.