consider replacing Zameck Guildmage with Varolz, the Scar-Striped .

varols works well with Lotleth Troll and can vastly improve Dreg Mangler 's scavenge cost (ind addition to adding scavenge to everything)

I was actually considering taking out the Lotleth Troll for another Zameck Guildmage and Dreg Mangler . The second ability of the mage couple with undying is huge. The regeneration of Lotleth Troll is nice but I usually don't end up sacing creatures to it. I also don't think scavenging is really that effective given it is only sorcery speed. I usually have better uses for the mana. In my experience, if I am getting late enough in the game that I have a bunch of spare mana to scavange, I've already lost.

Not every draw is a good one, quite often you'll draw a land or something too expensive too cast, there's nothing you can do to prevent that, but having options to carry on the good fight makes a deck more capable to cope with such situations.

In my deck i'm running 2 Zameck Guildmage 's and 2 Varolz, the Scar-Striped .

Zameck Guildmage gives card advantage, and keeps undying creatures on the board, but requires there to be a significant board presence in your favor to be of any use. He does nothing if you have no creatures available for him to target.

Varolz, the Scar-Striped allows you to scavenge cards out of your graveyard, allowing your graveyard to function as a second hand of sorts. And it can use it's regenerate ability to sacrifice undying creatures in order to untap and power them up, or for morbid, or to have a counter on them for zameck or to simply prevent his own death. He of course requires creatures to sacrifice, or he's gonna get killed fairly fast, and he's best to play once your graveyard has been filled up.

They both have their upsides and downsides, why not run both.

the deck i mention is:

BUG's up inside ya - Undying Aggro

also you should look into siding Duskmantle Seer , i had in my main for a while, but more often than not he would be sided out, because he is terrible against an aggro deck, which are fairly common in my meta.