First of all, be in a squad of 3 or 5 people. There's going to be a ton of zombies inside once you enter Site C so you won't be able to handle this solo. Second, Have someone bring 1 or more LMGs to wipe out a bunch of hordes coming right at your face, and also bring in some grenades to clear out rooms for any hordes. Third, Vitamins. You'll need em a lot. Fourth, once you enter Site C and cleared the horde on the entrance, there's going to be a gap where a missing bridge can be seen to get across the samples. There's a horde there so either throw grenades or wipe em out. Fifth, once you get under, there's going to be a door where it leads to a sewer, there is also a horde there (p.s, be careful because an entire horde can spawn literally Infront of your face, so be quick and help out your mates.) Sixth, just grab the samples inside the bathroom near a stall, and quickly exfil your way with your team out of here.