A) Bunkers, Missle Turrets, Perdition Turrets, Psi Disruptors and other structures don't count towards your unit count.
This is something that some people would look at and say "yeah that looks pretty obvious" but don't take advantage of the fact that you can literally fill the surroundings of your base with them. They're also insanely sturdy and just having 2 SCVs near it can seriously hinder an enemy's assault (judging from playing on Normal difficulty). Utilizing Perdition Turrets over Firebats is not only cheaper, but allows you to have more open slots to make more useful units.
B) A good 50% of the Mercenaries straight up aren't worth the credits or the resource/unit cost to buy.
I realized quickly in my first playthrough that Mercenaries were just a giant money/resource sink, and the lure was just too great in a bind to waste all of your resources just to have a slightly above par army for twice the resource/unit cost. The money spent on most Mercenaries really is better spent upgrading your units/structures. In my opinion, only the War Pigs, Spartan Company, and Siege Breakers are really worth it, and that's because both WP and SC can provide excellent support if you've been comprimised and the Siege Breakers get a devastating 66% boost on top of the unit upgrade boost.
C) You can group your units by selecting the group you want and right clicking one of the empty green boxes with a number near it on top of the toolbar.
This is a mistake I made throughout the first playthrough and halfway through the second that made me pretty proficient at commanding with my mouse, but made the utilization of things such as the Hive Mind Emulator and the Specialists impossible to use until I learned of it. Though the following guide will practically require you to have mastered this trait, it actually does not utilize it during the "All In" mission.
D) Don't shirk upgrading, and always upgrade the trait that you researched for the bonus effect first.
Upgrading was something I didn't really do before until I hit unit capacity. Which is not only unwise, but is a titanic waste of time and resources. Keep in mind that the biggest cost of these upgrades is normally time, and the benefit it grants will exponentially increase the battle power of your forces.
Now for the mission, "All In" the game has you make decisions which decide the difficulty of the battle. This is done through which section of the zerg force you try to decimate. If you choose to destroy the air forces, then the only real threat will be creepers in the air and you will instead have to find a way to deal with Nydus Worms both behind yours and enemy lines in order to lift the burden on your ground forces. If you choose to take out the Nydus Worms, then there will be a pitiful amount of enemies in the ground, but a significant amount more in the air, including several waves of aerial forces that bee-line it to the artifact.
I personally find that eliminating the Nydus Worms is the most crippling thing you can do to the Zerg simply because:
A) The game supplies you with Goliaths and Thors which have excellent anti-air abilities without sacrificing dominance over ground forces.
B) Unlike Nydus worms, the aerial platform can only drop a small number of zerg behind your lines
C) You can build an infinite number of Missle Turrets anywhere around or in your base. period.