Yea, it's always helpful to practise with the skirmishes (which are essentially combat-only versions of previous story missions) until you can ace them reliably with top rank. Generally speaking you need a solid core strategy to use (ie, I myself used a pronged blitz attack with Welkin and an Engineer and Scout in the middle, with Shocktroopers on the other side and Alicia on the other). Also, remember that the game WANTS you to take Alicia (not that she wasn't the most powerful Scout in the game by a huge margin anyway), Rosie and Largo along, since each of them also grants an extra command point whenever on the field. Besides them the must-have unit to get later is Marie, the best sniper who can eventually reliably get 100% accuracy for her shots.
Couple of tricks to know:
1) Welkin's mortar is good for clearing cover, you can then use a Shocktrooper or Scout to mop up the enemy.
2) Always position sniper support so they need to move minimally or not at all for rest of the mission.
3) Welkin can also crush cover with his tank, which is also useful for flushing out the enemy.
4) Aces can't dodge as well if they're unaware of the character.
5) Always bring along at least one Engineer.
6) Any unit can destroy an enemy tank with a Command active. Just rush behind it and have the Shocktrooper fire with Demolition buff at the radiator.