I need Help getting the last two achievements pls anyone

I only need two more achievements on halo 3 to fully complete the achievment list to date, they should be relatively easy but I just can't seem to get them, they have elluded me for the past several months and it is pissing me off, the two achievements that I need are
have fun respawning = get an extermination on a mythic map (Kill entire enemy team, overkill minimum)
save this film = get a perfection medal on a mythic map (slayer game type, 15 kills no deaths minimum)

if anyone can and is willing to help please and I will help you get any achievements that you are missing too ^_^

go beg kapowski! he likes playing games over and over and over, hell, he'd probably start the game again for you just to take you thru the lot to say he had!

Both of these two achievements are based online, it's possible to get it sorted in one slayer match if we can get 8 people together on team mythic it should be simples