I've had one since about week 4, but totally forgot I had it. I see everyone is dying for it, so I'm wondering if it's worth upgrading :)
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Having Praxic and the upgrade for "2 grenades" makes you a human grenade launcher with no reload. And with the Melee=Shield upgrade recharging super fast too, you can pretty much melee for a new shield every couple seconds. Between both of those, you are a force to be reckoned with in those few seconds of Radiance.