General Strategy
Use the third pre-made set, Safeguard: Defense Focused, with Barret and Tifa as your hero characters. The main reason for this is that you need a way to defend a lane without spending too many resources. The Ballista solves that need spectacularly.
Start each round by placing a Ballista down by the tower of the threatened lane, then create an Elite Enforced to hold the line with a Cleric to help keep them alive for as long as possible. Usually, at this point, you’ll want to drop another Ballista down in the same fashion on a different lane, depending on which one your enemies try to hit next.
Your main strategy is to go heavy on one lane — usually wherever you deploy first — and to the bare minimum to protect the others. Don’t overspend ATB on Enforcers. They don’t help you build up towards eventually calling in Barret or Tifa. Summoning them at the perfect moment for your final push is your ultimate aim. If you are facing a ton of green ranged units in a lane and absolutely must, hold out for an Elite Enforcer backed by a Cleric instead.
When deployed, Barret fully heals nearby units and fires a massive shot straight ahead, so be sure to get as much use out of him as you can with both abilities. (Remember that Hero Units do not cost ATB, meaning you can drop them simultaneously as a regular unit for a big push.)
Don’t neglect red Vanguard units, however, since you need to deploy enough to call in Tifa eventually. When deployed, she delivers a damaging dive kick to the surrounding area, and her Haste aura speeds up units. If you apply it to a tight squad deep in enemy territory, they can quickly sweep through everything. Also: don’t be scared off by troop disadvantages with your red units, assuming you have enough units of other types to back them up.
Now let’s tackle more specific strategies.
Hard Challenge 1
Open with a Ballista, Elite Enforcer, and Cleric in the right lane. Then pivot to the left land and go hard there for most of the match. Start with Rangers to deal with the incoming onslaught of blue units, then add in an Enforcer in the left lane to deal with the sword-aligned mech.
When able, deploy an Elite Vanguard to the right lane and wait to deploy another on the left side. From here, you should be able to pile Rangers and Vanguards on the left lane to eventually summon Barret and Tifa for your final push into the enemy stronghold.