The question here is how they revealed the objective so quickly. The main problem I've found with Mansions of Madness is that it doesn't give the Keeper enough info on how to run the game, which you have to figure out for yourself.
In the case of the first scenario, EVERYTHING should be geared to getting a maniac out and stealing samples as fast as possible, then forcing investigators to chase him rather than seeking the objective. The zombies can also be used in the same fashion, or be used as "blockers".
If the players don't stop the sample carrier, you can quickly build up enough threat that you can simply waylay them with Mythos cards.
One advantage you have is that by combining "Steal Sample" and "Command Minion", you can move 3 spaces on any turn where you start off sharing a space with an investigator, which allows you to outpace your foes and usually put a wall between yourself and any bullets, and that in the early turns you can gather Panic and Mythos without spending threat.
Combine this stuff with a continual pounding on their weaker stat (generally sanity for men, and health for women), and you can really slow them down.
Break a leg. No, literally, get them down to 4 health, and break their leg.