Ok, here is my guide to getting all the meta game acheivments.
Step 1:find some skulls which don't affect your game,for me these are fog, tough luck and catch if I'm fighting the flood. Step 2:use the weapons which give you medals (needle, plasma pistol/power drain and any headshot weapon) step 3:Do the levels quickly, getting the 3X multiplier is your top priority, don't worry about killing everything in the level, just make sure you get that time bonus.
Step 1:find some skulls which don't affect your game,for me these are fog, tough luck and catch if I'm fighting the flood.
Step 2:use the weapons which give you medals (needle, plasma pistol/power drain and any headshot weapon)
step 3:Do the levels quickly, getting the 3X multiplier is your top priority, don't worry about killing everything in the level, just make sure you get that time bonus.