It wasn't until I hit high gold, however, before I finally learned how to use them in combat. I had seen them in progames, I had seen them in crazy surround tactics, I had seen them to block ramps and bunkers from being repaired... Not until I did some unit testing, however, did I learn their true value in combat.
here is my guide, take everything I say with a grain of salt. If I forgot anything, just let me know, I'll think about what I do, and add it once I'm sure.
For illustrations with letters: Spaces are added for visual clarity, make them hug your units. Z = Zealot, S = Stalker, E = Sentry, z = zergling. Will be adding pictures as soon as I have time... Not sure when that will be...
BASIC TECHNIQUES
Forcefielding ramps:
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There are two ways to do this. You can just forcefield the very bottom in order to completely prevent damage. The other option is to wait until a small part of their army is halfway up the ramp, and then forcefielding the back of the ramp behind them, allowing your zealots and stalkers to kill them, slightly weakening their attacking army.
Preventing Kiting:
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Forcefield behind the section of the army you don't want kiting you, then kill it off. If your forcefielded more than you can take on, put two more down in front of them in order to prevent them following you, and retreat.
VS. TERRAN
Zealot-sentry vs. Small bio force:
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If you think your zealots can kill his MM without being evaporated (I.E. Before Stim/Conc shell) Make a line of forcefields, or just two (creating a very narrow choke) like this:
FFFFF FFFFF
MMMMMMMMMMM
ZZZZZZZZZZZZEEEE
Or
FFFFFFFFFFFFFFFFFFFFFFFF
MMMMMMMMMMMMMM
ZZZZZZZZZZZZZZZZZZZZEEEE
To trap their units to kill their force, and/or get some free units as they try to micro past the choke you have created.
FFFFF FFFFF
MMMMMMMMMMM
ZZZZZZZZZZZZEEEE
Or
FFFFFFFFFFFFFFFFFFFFFFFF
MMMMMMMMMMMMMM
ZZZZZZZZZZZZZZZZZZZZEEEE
To trap their units to kill their force, and/or get some free units as they try to micro past the choke you have created.
Zealot-Sentry-Stalker vs. Midsize Bio Army (With Stim/Conc Shell). You are going to want that golden 3:3:1 ratio (3 zealots and stalkers for every sentry).
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Basically throw down random forcefields, preferably connected (like walls) in the middle of their bioball. This will allow your zealots to get closer (even without charge) without dying, will prevent kiting, while still allowing them to kill everything in the front, while protecting your stalkers and sentries from getting killed by lowering the number of units behind the forcefields that can hit them. This can REALLY change the outcome of a battle if done right before the engagement starts. If you still have energy after the zealots clean up the front line, throw up guardian shields, move the zealots back, and move up the sentry/stalker to eliminate one line of MM at a time, as the forcefields should prevent the bio army from moving up, if they are still up. As soon as they feel they are going down, retreat unless you think your remaining forces can destroy the, (don't forget the move the zealots back to the front if you decide to re-engage after forcefields wear off)
Zealot-Collosus: Very large bio army: All upgrades and medivacs: This should only be your first push, as for your second one, they will have added vikings, and you will want stalkers. This is assuming no zealot charge.
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Before you put down your forcefields, put up guardian shields and move up your sentries to cover your engaging zealots. Then use any leftover energy to forcefield in the same fashion as vs. the midsize ball, allowing your collosus to retreat should all your zealots die, or to outrange things trapped behind forcefields. If your zealots have charge, put down the forcefields ASAP, before the zealots can be evaporated, but save energy for the guardian shields.
Stalker Collosus: Very Large Bio Army: Upgrades, Medivacs, Vikings
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Guardian shields then forcefields all in front of the bio army. Then retreat then turn back in order to have the maximum collosus-stalker range to minimize damage taken. If they are microing their vikings to snipe collosi, micro collosi to the back, and sacrifice stalkers in the front/side accordingly to distract/take out vikings.
Stalker/Zealot Collosus: Very Large Bio Army: Upgrades, Medivacs, Vikings... You want to use this composition vs. less vikings and more marauders
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Follow Midsize army guidelines, move stalkers slightly back before a-clicking with them to get maximum range and minimum damage taken, don't forget the guardian shields.
VS. ZERG
Vs. Zerglings: (this is mainly during a 3 gate expand) With Zealot-Sentry or Zealot-Stalker-Sentry
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If you have mainly sentries, create a wall in front of the zerglings and retreat to your ramp/natural. If you have a good force of zealots too, then forcefield in whichever way it takes to allow your zealots to engage without getting surrounded. For example.
zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz
zzzzzzzzzzzzzzzzzzzzzzzz
FFFFFF ZZZZZZZZZZZZZZZZZ FFFFF
FFFFF EEEEEEEEEEEEE FFFFF
FFFFFFFFFFFFFFFFF
If you have not microed your zealots to the front then...
zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz
zzzzzzzzzzzzzzzzzzzzzz
FFFFFFFFFFFFFFFFFFFFFFFFFFF
FFFF EEEEEEEEES SSSSSS FFFFF
FFFFFF ZZZZZZZZZZZZZZZZZZZZZZZZ FFFFF
This disables the zerglings from getting a surround on your zealots, while keeping the sentries safe.
zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz
zzzzzzzzzzzzzzzzzzzzzzzz
FFFFFF ZZZZZZZZZZZZZZZZZ FFFFF
FFFFF EEEEEEEEEEEEE FFFFF
FFFFFFFFFFFFFFFFF
If you have not microed your zealots to the front then...
zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz
zzzzzzzzzzzzzzzzzzzzzz
FFFFFFFFFFFFFFFFFFFFFFFFFFF
FFFF EEEEEEEEES SSSSSS FFFFF
FFFFFF ZZZZZZZZZZZZZZZZZZZZZZZZ FFFFF
This disables the zerglings from getting a surround on your zealots, while keeping the sentries safe.
Vs. Zergling/Roach with Zealot-Stalker-Sentry with superior forces
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Forcefield behind you to prevent zerglings from getting a surround on your units (MAKE SURE YOU MICRO ZEALOTS TO THE FRONT BEFORE GOING ONTO CREEP!!!) Then forcefield behind the roaches (if your army is big enough to defeat them) while guardian shielding. If you don't think you can fight them, kill the zerglings, then forcefield in front of the roaches while you retreat. This allows you to get a very good number of zergling kills for minimum army.
Vs. Zergling/Roach with lesser forces (for free kills) Zealot-Stalker-Sentry:
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When the zerglings run in front of the roaches, forcefield behind them and your units so they can't surround you, and then you can get a large number of free zergling kills (remember, ZEALOTS IN FRONT!) while getting a free retreat from the roaches.
Vs. Roaches, Medium Numbers
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Forcefields in the middle of the ball/front line, see vs. terran bioball midsize for mor information on why.
Vs. Roach/Hydra, medium numbers, Zealot-Stalker-Sentry:
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Forcefields in front of the line, kill off roaches with stalkers, then bring up zealots and have them attack as forcefields go down, as they rip through hydras. If they try to retreat, throw down forcefields blocking their retreat path, (surrounding them with fields) to get free roach kills with your stalkers (DO NOT KEEP ZEALOTS IN FRONT WITH THIS)
Vs. Mass Roach/Hydra or just Mass Roach: (DO NOT INCLUDE ZEALOTS IN THIS MIX) Stalker-Sentry Collosus
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Random forcefields spread throughout the roach/hydra line and front to force the roaches to reposition and allow less of them to fire.
That is all I have. If I missed something, please tell me. If you were helped by this (I hope you were, I spent 20 minutes on it) Then please reply to say thanks to keep it on top of the thread list so others can see it and be helped. Thanks all!