@Bartley: You're right, in this case I either don't play creature cards or I don't cast them for it's dash costs, so that I can use them as usually in the next turn.
@ganjaeone: You're right as well, Forerunner is worth playing, if I have at least two other creatures on the battlefield. But one third of my cards are creatures, so that's possible and it can get even stronger and has trample. Purphoros is a great deal!
@Lumprado: Even you are right,
Warstorm Surge fits perfectly but has high mana cost, so
Impact Tremors and
Electropotence do basically the same job and Purphoros will do as well.
Thank you for your ideas!