Mass Assembly is a lategame strat, which is vulnerable to early aggression.
When it hit's it's indeed very hard to defend (and for most people impossible).

What you need to do is, when it's already too late:
- Seperate the army (Shockwave, Replacment earth, quick freeze, etc.)
- Get ton of DPS units (BAFUs are your best friend)
- Get some Debuffs going (Chunders, Cultists, gargoyles)
- Ember Demons can meat shield and does good AOE on death
- Focus killing the Colossus first, otherwise you just feed into the zycle


Against Ember Demons, you need Ember Demons by yourself.
Just scout them and send them into the AIs Ember Chamber.

Good points above. If you are doing the Sloth Tree (On the right,) there is an icy defense that when activated, destroys Ember Demons. I think quick freeze does the same too?

I wonder that since Gold Sentinels are made from metal, Gold, and it being highly conductive, If the monster from the lightning potion would do any extra damage. Some niche thing, like a Werewolf being killed by a spoon, because it's, "Silver-ware."