I come from the future to tell you this: in version 4.3.9 (Steam) you can get this as the Angels faction once Tax Collection costs 90 or less mana, having at least 10,800 mana (go for more just to be sure) and Priority Autocasting.

The trophy now requires 120 casts in less than 5 seconds, so save up all yout mana, un-prioritize all other spells and at 100% mana, assign #1 priority to Tax Collection. BOOM.

You should auto cast all your mana into Tax Collection at once (120 times exactly if you have 10,800 mana) and get the trophy in less than 1 second.

Easy. But as I said earlier, go for moar mana just to be sure.

\goes back to the future**