Kog'maw Ultimate(R) update idea

I thought of a change to Kog's ult that might fit his identity (thematically and as a siege marksman) better.

Living Artillery 50 mana at all ranks (maybe 70/50/30 if his early game needs the restriction). Stores charges (3 max) with a 7(?) second cooldown between shots, with 45/35/25 seconds to generate a charge.

Range unchanged, AoE size unchanged

50/100/150(+0.1AD)(+0.2AP) on impact. Leaves behind a growing AoE for 5 seconds (similar to Cassiopeia's miasma). Enemies that enter the pool take (unsure of the numbers here) and have their armor and magic resist reduced by 5%(stack-able, one stack per second of DoT) per second as long as the pool exists.

Right now Kog's ult is more of a single artillery shell that nukes a small AoE, but I feel that doesn't really make sense, since its called Living Artillery. I think that it shouldn't be a nuke, but more of a delivery system for matter devouring micro-organisms from the void that live inside of Kog. And the organisms are short lived outside of Kog, so they have a very short life span, hence the DoT and AoE ending at the same time, since the DoT is the organisms transferring from the ground to the enemy.

And the armor/MR reduction should last beyond the duration of the DoT, like 2 seconds, since it takes time to recover from having your defenses eaten away (assuming all items can magically repair themselves), and if the enemy makes the mistake of it, or you somehow force it, taking all 15 seconds of the damage would result in a 75% reduction to defense.