Into the Breach is an awesome turn-based tactical RPG with lots of depth crammed into a simple-looking game. It really puts the strategy in SRPG, doesn't it? With these tips, you'll hopefully have more success on your next hard difficulty run.
Control the center
At the start of each game, you should usually put your mechs in the center. On maps where the center is obstructed, place them wherever you can protect all of your cities. Artillery units like rows and files where they can cover the whole map so think carefully about where your starting positions should be.
If there are many Vek on the board and more burrowing next turn, you should be very worried. Good players don't think of only the current turn but how to minimise the damage that they'll take on future turns. This leads to my next tip.
Kill Vek ASAP
You need to take whatever opportunity you can to kill Vek. Vek will die from taking damage from your or other Vek attacks, bumping into walls, drowning in water, lava, or acid, being on fire, or falling into chasms.
Block spawn points
Blocking spawn points stops the Vek from coming up but only temporarily. You will still need to block it next turn and it's likely that more spawn points will appear as well.
Protect the grid
When you can't kill Vek without taking grid damage, simply opt to protect the grid.
Look for opportunities to address multiple Vek with one action. The Rift Walkers' Artillery mech is one example of a mech that can push multiple enemies with one attack.
Push Vek into water, lava, or acid. Ground-based Vek can drown but both massive and flying units can't. You can also get the Vek to run into water for you sometimes.
Use hazard tiles
Some missions have tiles that will turn into water or lava on their turn or even kill any unit standing on that tile instantly. Naturally, you want the Vek standing on those tiles.
Exploit Vek attacks
Look for situations where the Vek will fire on other Vek. You can stop an attack on one of your cities by using another Vek to stop them but make sure that the Vek will actually fire in the order you want them to. If you didn't know, you can check the enemies' attack order by hovering over the icon in the top-right corner of the screen.
Check valuable info
You can check how much damage the Vek will do to any target by hovering over the Vek; simple, right? The big yellow arrows also show if the target will get pushed. Make sure that they don't get pushed into your cities.
Sometimes, the only way to save your grid is to block the path of enemy projectiles. Use your mechs to stand in the way or push another Vek there instead.
Some missions give you unique opportunities to kill Vek such as the terraformer on R.S.T. and the acid launcher on Detritus.
Use smoke wisely
Any Vek standing in smoke will have their attack canceled. If you're playing the Rusting Hulks squad, you'll have lots of opportunities to cover your enemies in smoke.
Use fire wisely
Fire is the mechanic of the Flame Behemoths but it also occurs when forest tiles take damage or after standing in lava in the final mission. Burning enemies take 1 damage before their attack phase. Remember that whenever enemies die to fire, they can't attack the grid. If a burning enemy has 2 HP and misses their next attack, they will die before they can deal any damage because they will take 1 damage at the beginning of their next 2 turns.
Use acid wisely
Acid is used by the Hazardous Mechs but is also found pretty much everywhere on Detritus. Acid causes the targets to take double damage to attacks but the damage from bumping walls isn't doubled. Acid also removes armour and it doubles self-damage so you have to be extremely careful when playing Charge mech, Leaper mech, or Unstable mech. Nonetheless, it's a very potent tool for you to deal more damage.
Use freezing wisely
Enemies in blizzards on Pinnacle become frozen or you can use the Cryo Launcher on the Ice mech. Frozen enemies become completely immobilised yet your own mechs can escape by using their turn but it's best not to get frozen. You can strategically take damage while frozen to become unfrozen. You can also freeze cities to protect them from damage.
Get targeted strike and smoke drop
Targeted strike is a simple ability that lets you deal 1 damage anywhere on the map and pushes adjacent targets once per battle. It's a potent ability because it can push any target no matter where they are. Better yet; it costs no cores! Put it on your least mobile mech like your combat mech when playing Rift Walkers. Smoke drop is the smoky equivalent to targeted strike and you can cover a 5-tile area in smoke with it. You will cancel any attacks of Vek standing in the smoke and it's an awesome tool for emergencies.
Upgrade movement speed
Attacking and repositioning Vek when and where you need to is the name of the game. Movement speed is very important no matter which squad you're playing.
Deal with scions accordingly
Don't worry about scions if you don't have to. Soldier scions are easily ignored but usually not blast scions. You have to judge for yourself if you can safely ignore the scions.
Consider enemy types
Think about which islands to tackle as some squads aren't able to deal with some enemy types effectively. For instance, burrowers will make you cry as Flame Behemoths or Steel Judoka.
Consider mission types
Think about which mission types to tackle. For instance, some squads deal low damage so they struggle with "Destroy X Enemies" missions. Some squads have great positional play so you should choose missions with water.
Reposition your mechs
Attack your mechs to reposition them. For instance, your Artillery unit will sometimes get webbed and there's nothing you can do but fire an attack on it to push it out of danger.
Use blast scions to your advantage
When blast scions are on the field, Vek explode for additional damage so make sure Vek die next to each other but not next to your buildings.
Mech damage isn't crucial
Don't worry too much about taking Mech damage. As long as they're not dead or on fire, they can keep fighting.
Final mission tip
Block enemy spawns during round 1 part 2 of the final mission and a fireball will kill the Vek as they emerge.