I presume you mean the MK competition (fastest vs race on Mario Circuit (which is modified))
It depends which pick ups... You cannot block the lightening bolt or the blue shell, so if you get hit by one of them in the race you have no chance of getting under 1:40 (my time is 1:39.200ish) The POW you can decrease it's effect by doing an upward flick at the right time.
Red shells of course can be stopped by simply holding a green shell or banana behind the kart (or a chain of three bananas) so you should always have one held behind you and one in reserve. fake item boxes are useless though, so just drop them)
My tactic is to start fast, and get a draft (slipstream) from the nearest kart which bring me to about 4th/5th before the item boxes, if I can pick up an item in 4th/5th then there is a chance it will be a mushroom or even three, if so then I am in luck and I can then use it/them for a nice shortcut across the grass, the rest is just good racing. (although if you really want a good time you may as well quit if you get blue shelled/lightning, which in this competition isn't too often thankfully (about 1 in 4 races you will be blue shelled/lightning I think)
