In short: I would hope my opponent to be dead before I even had the opportunity to play Mikaeus. And enough of my creatures have undying that it's added effect is a little superfluous.

24 mana is to try and get Phyrexian Obliterator out ASAP. Grimgrin of course already provides a base semi-late game heavy hitter.

I usually advocate some of my sideboard to change general deck dynamics slightly in my favor based on my opponent's deck, rather than have specific pointed counters. I've found this to be very helpful. For example:

Right now, the butcher ghoul is the good mid-level versatile card. It gives me blockers, can be used on offense, and combos well with the other cards.

If I'm going against a deck that has cards of particular inconvenience to me (say oblivion ring or whatnot), or a very aggressive deck, or a control deck, I'll switch it for the Black Cats to possibly foil their plans. A long game opponent simply means a switch of 1 diregraf ghoul to 1 mikaeus.

If I were going against Humans (to include Mirran Crusader), the deck would look like this:

15 swamp

1 island

4 turn to frog

2 geth's verdict

0 altar's reap

0 butcher ghouls

3 Black Cats

1 Grimgrin

1 Mikaeus