We don’t give you specific in-mission tips for beating the game in Heroic or Legendary modes; such tips would be repetitive, as they are essentially the same for every mission. Why? Because the real answer for how to beat the game in Legendary mode is “play faster with fewer mistakes.”
That said, here’s a look at some metrics on the four modes, including a discussion of the specific skill set the design team wants players to develop for each mode. Please note: this information comes directly from the team’s documentation, so you’re getting it straight from the horse’s mouth here.
Some Common RTS Terms
The acronym APM is a gaming term that stands for “actions per minute.” In general, the higher the difficulty level, the more APM you must perform. Beginners can usually perform about 20-30 actions per minute in a real-time strategy game. On the other end of the scale, professional e-gamers average around 300 APM, and often go over 400 in particularly intense battles.
Another common RTS term is “expand” or “expansion.” This refers to the practice of building secondary bases around the map in addition to your main base.
The following notes also refer to “rally points,” which are simply gathering points that you can manually set on the map for each base. When any new unit is produced at that base, it emerges and automatically proceeds to the rally point. Setting rally points in the right places can give you certain tactical advantages.
An important and ubiquitous RTS term is “counter.” Certain units are designed with abilities that make them more lethal to specific enemy unit types. For example, the UNSC Wolverine has a rack of deadly anti-air missiles that can decimate Banshees or other enemy flyers. Thus, when Banshees appear, you want Wolverines as a “counter” to them. Knowing your counter-units is an important skill in mastering RTS gaming.
The term “micro” (used as a verb) refers to the management of individual units or unit types within the course of a battle. The example just above is a good one: Banshees suddenly swoop out of the fog of war, so you quickly group-select your Wolverine units and direct them to concentrate fire on individual Banshees one by one, thus tearing apart the flyer squadron swiftly. If you don’t “micro” your units, they may self-select targets they don’t counter as well.
Finally, you’ll see regular reference to the term “economy.” This refers to three basic aspects of RTS play:
- The accumulation of resources—in this case, of Supply and Power—that can be used to build, produce, or buy stuff.
- The construction of physical facilities at your base—Turrets, Generators, Supply Pads; Garage, Armory; etc.
- The production (sometimes called “training”) of your combat troops and other units.
The Modes
Easy
Leader Powers
There is no requirement to support units with offensive Leader Powers. You should still heal/repair your units occasionally.
Upgrades
You are not required to trigger upgrades, although upgrades are advertised and incentivized in-game.
Controls/APM
You are not required to use global or local rally points and should be able to tackle most encounters by group-selecting All Units (the “big ball of death” tactic) and moving from threat to threat. The need for high APM is rare.
Average APM
15
Economy
You need to construct both Supply Pads and Generators, raising inflow to a stable level and then expanding in order to complete missions comfortably. Collect some on-map resources too (Supply and Power pickups), particularly in missions with minibases.
Base-Building
You should expand, and you’ll benefit from the added economy.
Enemy
The enemy neither expands nor provides much harassment of your bases. The enemy won’t cast Leader Powers to support their troops outside of mission-scripted Powers.
Elevation Advantages
You won’t need to use these.
Special Abilities
You won’t need to use these.
High-Level Behavior
At a basic level, you can simply accumulate an All Units “ball of death” and rampage around the map, moving from threat to threat. You can get to locations manually via the basic camera movement without using the Fast Camera move. Finally, you should be exploring the use of the global rally point to help deploy your forces more tactically.
Normal
Leader Powers
Required to support your units with healing/repair and the occasional offensive Leader Power. No requirement to invest in passive Powers.
Upgrades
You should be upgrading some units and using available upgrades at the Armory.
Controls/APM
Use global and local rally points so your troop movement is more automated. This helps reduce the need for high APM.
Average APM
30
Economy
Aim to capture some Power Node capture points on the map to support your economy.
Base-Building
Expand when possible, as you’ll benefit from the added economy.
Enemy
The enemy will expand and also capture Nodes from time to time, harass your base quite often, and occasionally use random Leader Powers in addition to mission-scripted Powers.
Elevation Advantages
Useful in encounters, but not always necessary.
Special Abilities
Use these occasionally in encounters.
High-Level Behavior
At a basic level, we expect you to accumulate forces and start to understand counter-units. This should lead to splitting forces manually by double-tapping 
to select units of a certain type and then targeting the specific enemies those units counter.
Use a combination of the Local Units and All Units commands to separate forces and apply appropriate forces to counter situations. However, we’d expect you to fall back on All Units to get out of trouble in a pinch.
Use the D-Pad shortcuts and the Fast Camera move to reach threats quickly. Also use the D-Pad to cycle through bases and units.
Set global rally points to auto-send troops to threat locations.
Heroic
Leader Powers
Required to support your units with frequent healing/repair and frequent offensive Powers. You should begin to combo these Leader Powers.
Upgrades
You should be upgrading some units and using available upgrades at the Armory.
Controls/APM
Set global and local rally points as an efficient way to micro your units directly to more threats.
Average APM
60
Economy
Aim to capture most of the Power Nodes on the map to support your economy.
Base-Building
Expand whenever possible to boost your economy.
Enemy
The enemy expands more aggressively, tries to capture Power Nodes often, and harasses your bases frequently. The enemy also casts the occasional Leader Power.
Elevation Advantages
Use these in all encounters where available.
Special Abilities
Use these frequently to gain combat advantages.
High-Level Behavior
At a basic level, we expect you to accumulate forces at an accelerated rate, split and manage multiple forces easily, and have a good understanding of unit counters and how to apply them quickly in fluid situations.
Heroic players need to use the D-Pad shortcuts to quickly jump to threat situations and/or cycle through bases and units, and use the Fast Camera move to reduce downtime and increase APM.
Heroic players must use the global rally point to assist threat locations, and use the local rally point to separate off reinforcements and support capture points or the enemy’s harassment operations.
You can still use the All Units command when situations get out of hand, but if you employ effective micro commands, you won’t have to rely on it as much.
Legendary
Leader Powers
Use these whenever available to support your units with regular healing/repair and frequent offensive Leader Powers. You must learn to combo your Powers and invest in passive Leader Powers in order to bolster your economy.
Upgrades
You should be upgrading all units and using all available upgrades at the Armory.
Controls/APM
You should be using the global and local rally points to help reduce the need for high APM.
Average APM
90
Economy
Aim to capture most of the Power Nodes on the map to support your economy.
Base-Building
You must expand quickly, as you’ll need the added economy to deal with escalated threats.
Enemy
Your enemy aggressively expands, often seeks to capture Power Nodes from you, and harasses your bases frequently. The enemy also employs Leader Powers quite frequently.
Elevation Advantages
You must exploit these whenever they’re available.
Special Abilities
Use these frequently in encounters.
High-Level Behavior
At the Legendary level, we’d expect you to accumulate forces with great efficiency, have an excellent understanding of unit counters, and know how to split and manage forces by using the game’s hotkey mechanics to assign units of certain types to control groups.
You should effortlessly use a combination of the Local Units and All Units commands to separate forces and apply appropriate forces to counter situations.
Legendary players must use the D-Pad shortcuts and/or the Fast Camera move to reach threat situations quickly, reduce downtime, and increase APM. You should be masterful at using the D-Pad to cycle through bases and units.
Finally, by now you should have a healthy fear of the All Units command, because it can undo all the careful unit allocation you’ve accomplished and potentially throw a wrench into your meticulous unit placements. However, Legendary players will certainly use the All Units command if a situation requires all their units.