This bull cost A LOT ! You need 5 regular formas + an Umbral one.
How to play it the most efficient way?
In the beginning of the game: take damages. The pick up heath orbs to stack [Health Conversion]'s stacks.
Cast your 1st ability as much as you can if there's a pack of enemies. (Otherwise, cast your Iron skin and wait for a good opportunity.)
cast your 2nd ability right after your 1st one to obtain the buff for [Ironclad Charge].

With this simple method you can easily reach +350K on your Iron Skin

The aim of this build is to TANK. You can forget about any Eidolon/Buff build after doing this one.
You don't need to upgrade [Iron Shrapnel] (you'd need a 2nd Umbral forma), it's only used to cast the ability.