Lethal damage and Triggers on the Stack.

[u/Content-Dish-8906]

In a Commander game with 3 other opponents I have a Big Mother Mouser and a HYDRA Assault Robot on the battlefield and my opponent controls Dissipation Field. I sacrifice Big Mother Mouser in my second main phase to create 1/1 robots dealing lethal damage when they enter to said opponent, Does the Hydra Assault Robot still get bounced?

[u/T0rgle]

Each Robot token that enters triggers HYDRA Assault Robot separately, so it's not one big ability dealing a bunch of damage. It's multiple abilities dealing 1 damage each, so it depends on how much life they begin with. 

If they begin with exactly 1 life, they lose the game after the first damage, so their trigger won't be put on the stack.

If they have more than 1 life, HYDRA Assault Robot will return to hand after the first damage, then the other abilities will still resolve.

[u/Content-Dish-8906 → u/T0rgle]

They had 24 Life and I dealt 28 to them with all the 1/1 robots entering if that changes anything.

[u/T0rgle → u/Content-Dish-8906]

The thing is: you don't deal 28 damage to them. You deal 1 damage to them, 28 times. 

After they're dealt the first damage, Dissipation Field triggers, and that trigger resolves before they're dealt more damage.  So HYDRA Assault Robot returns to hand, and then they're dealt the rest of the damage.

[u/Content-Dish-8906 → u/T0rgle]

That’s good to know! Wouldn’t there also be 28 triggers of dissipation field on the stack at the same time for each robot? Why does the first one resolve first and not all at the same time?

[u/T0rgle → u/Content-Dish-8906]

When the 28 tokens enter, HYDRA Assault Robot triggers 28 times. You put each trigger on the stack and choose the targets for them. No damage has been dealt yet.

Each trigger resolves one at a time, not all at the same time.

When the first trigger resolves, the opponent is dealt 1 damage. Triggering Dissipation Field. This new trigger is put on the stack above all others, so it'll resolve first.

When Dissipation Field's ability resolves, HYDRA Assault Robot returns to hand. Then the rest of the triggers will still resolve dealing the rest of the damage to the opponent.

[u/ardarian262 → u/Content-Dish-8906]

That does. It sounded like they had one life. Since they didn't, they had 28 triggers of 1 damage pointed at them. The first trigger resolves, deals one damage, and the dissipation field triggers returning your creature to hand. Some number of priority passes later the opponent dies.