i just finished the hard campaign with the ig
going to try beating campaign with the necrons now...
for the ig use Heavy weapons team for defence... ít's a must for the ig'
place them at corners so that melee units will have a hard time reaching them.
For a strong enemy (strongholds,players,large provinces)
make a baneblade two leman russ tanks a maxed out karskin squad (Attach a priest,upgrade with plasma weapons<-- optional_
and 3 maxed out guardsmen squads atleast 1 commissar is a MUST for an large offensive because there morale will be 0 after a few seconds (i suggest attaching a psyker cuz a infiltrated enemy can be a pain...)
then stop the 4 squads in 2 chimeras and march up with your baneblade,leman russ tanks and chimeras <-- this is pretty likely to fail at first try, so after sending this guys to battle,start making new squads to send directly after the first wave (2-3 of this waves will crush any enemy)
Some tips for the ig:
commissars can make from a normal guardsmen squad a killer-machine (always have more then 1 squad)
always attach a priest to a karskin squad, also you should have a back-up squad with a commisar: the commisar (special ability) will make karskins/guardsmen even stronger
for the dark eldar: when your at the point of *making a base* don't finish your hq just yet, wait till it's done for about 90% because the dark eldar won't start attacking till you have finished it! Then start capturing the 3 points+ set posts on them (you can even place some buildings next to it.
also, after destroying the dais, you have won. so make a very strong anti-verhicle defence the the 3th time he comes is your sign to kill it: he won't run away till he's at... 40% so let him come closer, and then surround it with verhcles that can take alot of damage, then you can probably kill it
(i won this way in 35 minutes xd)