I'm not entirely sure about that one. I'd love to test your deck, but I don't have enough Serendipities yet.
FWIW, here's my theorycrafting on brontobow vs Ghostmare:
Time towers represent a creature-based solution to Ghostmare. Most creature-based solutions fail due to Reverse Time, as I'm sure you've seen with your Otyugh. Your Oty gets rewound, isn't able to do its job, and the ghosts power straight through your shield and your 100 HP. If you are using time towers to play the ghosts, those will get rewound as well. 2 towers aren't going to produce enough quanta to dump the ghosts in your hand as well as the ghosts that were rewound into your deck. The towers would be wastes of space when you're not fighting Ghostmare, too, because your deck doesn't use
quanta except via Serendipity.
The first question is, what can be replaced? IMO, you can afford to lose 2 quantum towers, but I don't know how much quanta you typically burn via 6xSerendipity. I only have 2.As for replacements, you could use Lightning to kill the ghosts, helping feed your Bonewall when Oty can't do it. Lightning can help with dune scorps, too. You could take an Electrum Hourglass or two, at the risk of them being stolen and used by the enemy. A better choice might be Sundials, as they give you some respite from the damage rush as well as help you draw deeper into your deck past the rewound creatures. If ghostmare steals your sundial, you still get a turn of respite from damage, and ghostmare can't use the dial's draw power.
A backup, more reliable shield could also help - Fog Shield, Mirror Shield, Diamond Shield, Phase Shield, or Skull Shield could all potentially work, though for many of them you again risk having it stolen.
Here's a deck that commonly crushes my SoSac - YMMV since the enemy deck obviously has 2x draw, instead of the other way around. 24 turns of SoSac is very very good, whereas players themselves only get 12.
