der Amtsschimmel — the office horse. Duden: an excessively exact handling of official rules, bureaucratism. The word is used jokingly.
zerreden — to talk something to pieces, to discuss it so long that interest in it is lost. Ein Thema zerreden, to talk a topic to death.
die eierlegende Wollmilchsau — the egg-laying wool-milk-sow. It is something that is supposed to meet every demand at once, including demands that contradict each other.
der Hundertfünfzigprozentige — the hundred-and-fifty-percenter, someone who pursues something fanatically.
der Prinzipienreiter — the principle rider, someone who insists pettily on his principles.