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This an guide on the Fusion Grenade.
The Fusion Grenade is a glove that throws grenades. You start the game with this weapon, so it's not very powerful and quickly becomes less effective. However, it's very useful early-on.
Each grenade explodes one second after it's thrown or when it hits an enemy, so you can bounce the grenades somewhat. Throw the grenades at groups of enemies to get the most damage out each one.
If you use the Fusion Grenade enough you can upgrade it to the Fusion Bomb before Ardolis. You will probably upgrade it several times at the Nundac Asteroid Ring, as you will be using it a lot and dying many times too. In terms of Raritanium upgrades, damage and blast radius are what a grenade needs. The Concussion Detonators is this weapons special upgrade, but it's really not that useful. It knocks down most enemies to give you more time, but usually you will not need it or you will be able to throw another grenade at the enemy before they attack. Use your Raritanium on something else.