Yes, opposing creatures will get -2/-2 before your opponent gains control of Elesh Norn.
Elesh Norn's ability is a static ability that applies continuously.
604.1. Static abilities do something all the time rather than being activated or triggered. They are written as statements, and they’re simply true.
Coercive Recruiter's ability is a triggered ability. Therefore, its effects do not happen until the trigger resolves.
603.1. Triggered abilities have a trigger condition and an effect. They are written as "[When/Whenever/At] [trigger condition or event], [effect]. [Instructions (if any).]"
603.3. Once an ability has triggered, its controller puts it on the stack [...]. It remains on the stack until it's countered, it resolves, a rule causes it to be removed from the stack, or an effect moves it elsewhere.
With that in mind, here is how your specific situation unfolds:
- All creatures return to the battlefield from the graveyard under their owner's control.
- At this point, you control Elesh Norn and all of your opponent's creatures have -2/-2.
- Any creatures with zero or less toughness die.
- Your opponent chooses Elesh Norn as the target for Coercive Recruiter's ability, and then places the trigger on the stack.
- Coercive Recruiter's triggered ability resolves.
- At this point, your opponent controls Elesh Norn.