The diamond shard strike Sindri's enormous armor to little avail except making loud noises.

Sindri, in a rather spectacular motion of agility that is remarkable for humans let alone someone in armor that bulky, he jumps up, avoiding the strike of the earth elemental and brandishing his pistol. Before it can recall its arm, Sindri lands on it and begins firing at the water elemental, which will be stricken by destructive and very powerful bursts of energy from the pistol.
Sindri runs up the arm of the earth elemental and strikes it in the body with his glowing staff, which would shatter the elemental in a singular spectacular blow.
Sindri quickly retrieves his staff and points it between the air and fire elemental, which must now contend with the fury of a blast of pure immaterial energy.

After all four of the elementals are taken care of, Sindri will hover in the air and envelop it in a flurry of thunder and lightning, ionizing the atmosphere with such voltage that lightning arcs in between trees.