HATE. That is all.
A good finisher is Dovescape + Elesh Norn, Grand Cenobite . or something like it. Your best bet is infinite combos for an instant win. since you have access to black here are a few of my favorites: Triskelion + Mikaeus, the Unhallowed and Sanguine Bond + Exquisite Blood .
I got a Merieke-Deck too... ...love her.
card:Minamo, School at Water's Edge is the BEST card for Merieke.
If you play multiplayer, I would cut some counterspells... ...they are only good in 1on1s.
Mortify is nice.
There is some nice synergy with Merieke, if you can make creatures indestructible (e. g. Eldrazi Monument , Deathless Angel , That Which Was Taken , Avacyn, Angel of Hope ...), because Merieke would "destroy" the creature you got control of.... ...so you will keep them permanenty and they are indestructible, even if Merieke dies or gets unttapped).
Unbender Tine ist nice for Merieke too... ...and works as mana ramp sometimes.
If finishing off is the problem why not try a planeswalker like Sorin Markov ? Leaving an opponent in 10 life should do the trick specialy on life gaining decks or you can snipe off and then control the player for a turn (great flavour)
If not look for some painfull black spells, there are many that can hurt a LOT. You can check some in this deck I once tried to use them lol Shadow+Evil spells
i've got a merieke deck too and i love it to pieces. it's up on the site if you want to take a look.
might i suggest some other things that'll allow her to untap? card:Minamo, School at Water's Edge and Unbender Tine are great suggestions. i usually end up tutoring in a Freed from the Real if i can. Intruder Alarm is pretty fun with her too. going along with what Ghoul was saying; i put in Aegis Angel and Darksteel Plate which can both work wonders on either merieke or the creatures you take control of. there's also rare occasions i can go infinite with a Arcanis the Omnipotent and Mind Over Matter but that doesn't happen often.
I've added both dovescape and guile and am STILL pursuing a Minamo (someone I know has one and is willing to part with it but for the last several months it has been at his home, over a hundred miles from campus).
I don't really need more methods to untap merieke aside from the land and what I already have, and intruder alarm would force her to untap which is bad. Arcanis and mind over matter are both good, although i think if i don't win upon resolution of the latter I will immediately die. I don't think time sieve would be good enough as it would require a specific card to do anything, and I always feel bad about setting an opponents life to a certain number (usually ten) as it is not balanced for the format, intended to halve a total, not quarter it.
Deathless angel and avacyn seem to be the next goal cards aside from the obvious and persistent Minamo.
a really good way to weaken the enemy is use Magister Sphinx, Desecration Demon, Divinity of Pride or Ghastlord of Fugue.