You want to beat IG? You have to neutralize whatever his biggest threat is right away. Now this seems vague because it is. You have to look at his list and your own list and see what is going to mess you up the most. For example:
Your opponent has a Manticore and a LR Executioner. Both of these are huge threats, but you only have enough firepower to kill one a turn. Which do you try to kill first? At this point you have to look at your own list and see what could mess you up more. Are you mechanized? Kill dat Manticore before you're stuck in your DZ. You're running an Assault Marine BA list? Kill the Executioner first, your power armour laughs at AP4 even if it causes instant death.
This is the kind of thinking you're going to have to use to crack open the shell of IG. There is no one tool for doing this unfortunately aside from massed assaults, which 6th does not favour on the whole. Also: focus fire on one target until it is no longer a threat. Even shaking that Executioner removes its primary threat, but you do need to deal with it next turn. Hull points help you here as many guard lists rely on vehicles to do the heavy lifting. A librarian with Objuration Mechanicum can be especially annoying if only for the automatic Haywire hit.
With your Tau, use your railguns to reach out and touch him in his deployment. With TLoS,you should be able to find any hiding artillery units. Abuse your shrouded 3+ cover tanks to the extreme and try to limit the amount of fire incoming to your units. Getting into a straight up gunfight with Guard is a risky game that is stacked against you currently. DS your suits into his backlines will shift his fire, but they can be wiped in a turn of shooting fairly easily if they get straight up turned on by his entire army. A refused flank can help minimize incoming fire, but he'll probably still be able to range you.
With your Marine armies, if you roll Hammer and Anvil you're probably screwed. He'll probably deploy within 12" of the board edge you come to him. Depending on the mission however, you may be able to bait him into coming to you by placing objectives in his half of the board, but closer to the centre, forcing him to move from his (presumed) cover camping positions. This can be use to spring board assaults closer to his main lines.
Drop Pods are an easy way to get up and close. Just keep in mind that his entire army WILL try to kill those units that drop in his face. You can use this to your advantage by distracting him with these 'expendable' units while the rest closes in. The pod is also useful for LoS blocking. This could work with your BA. Drop the pods turn one- run and odd number, and never drop just one, so you'll want 3+ (one can be empty if you want) to be an actual threat- to drop a lot turn 1, hide behind the pods for cover, take pot shots, assault turn 2 if still alive. While this is happening, the rest of your army is charging forwards hoping to be in a position to threaten targets from turn 3 onwards. You will take heavy casualties, especially on the pod squads, but you are facing Guard on their best deployment, so you shouldn't expect a walk in the park.
Assault can also be a problem if you assault a blob squad with flamers and Straken nearby. 20-50 guys with 2-5 flamers + overwatch + counterattack + stubborn = brutal for any assaulting unit. Mind you, that unit will be 200-450 points depending on setup. Tying something like that up even for a turn or two can be useful even if you get clubbed to death. Just try to see what he's going for and try to limit his strengths.
Hope that helps.