[Lukeatus]

It's simple -
With 4 minutes left to go, zombies start respawning quick, so you'll have to kill them at 1 gate and go right to the other - do not stand around!!
With 2 minutes left to go, zombie respawning is nearly automatic - you kill one, another respawns. So you have little time - Always just kill the ones at the gate, turn around and go to the other. It's all you can really do.
If I don't have the disciplines I mentioned, I usually just do it with the Fire Axe and blood buff. The zombies killed provide me with blood to use (occult item). It's really tough, but I'm pretty good getting from 1 gate to the other.
I always do it right after I get to hollywood because I always want to get my firearms skill from 2 to 3 right away (you get it to 2 with the books you buy from Trip's Pawnshop). So, firearms 3 with no XP use.
It's a tough mission... I remember it being easy with celerity of course, and with a Ventrue I had dominate 5 - I got to a gate, had every Zombie suicide, than turned away, as soon you get to a gate you make them all die instantly. It's much better than having to hack through them. First time I played I thought it'd be a fun mission where you get to shoot helpless zombies; I quickly learned that it wasn't the case.
Just remember: It really isn't about how many zombies you kill, it is about for how long can you keep the gates from constant bashing (both of them).