Imperial Guard

The imperial guard, even though they're a bunch of human pussies, have alot going for them in the form of HUGE WEAPONS WITH TEMPLATES. When you face off with imperial guard they'll most likely roll out 3 Leman Russ battle tanks, a host of lascannons, missles, grenades, plasma, etc that will devistate you from the word go.(especially if they go first) The situation isn't hopeless though. Mekboys, force fields and a little luck can help you survive to dispatch the nasty little bastards with big guns.

Looter armies such as the deathskullz allow for you to compensate for the dakka of the imperial guard. What I recommend is atacking them where they keep their armor and using your basilisk and Leman Russ to take care of their more scattered squads. Their tanks have to die though. Let them live past turn 3 and you'll lose.

Cover can be a big help when fighting the guard so make sure you take advantage of all possible cover. Their bad shooting ability combined with better saves due to cover will make it alot easier for you to keep boyz on the table.

Template weapons are great for the orks against the imperial guard because they've got so many toops you can actually hit something. They've got bad saves so you can actually kill something. Don't burn up too many points on guns but templated guns do help against the guard.

Due to the large variety of army combinations in warhammer 40k for the imperial guard you should consider your list based on what they're coming with. If they're mechanized you'd better be ready to pop tanks. If they march out 50 different squads with rocket launchers you'd better be fast and get in before the end of turn two. Their diversity and large numbers is what has made them effective defenders of the Emperor.