Having recently completed 50/50 Sieges with 3 stars each (150/150 stars), I thought I would share some useful tactics / strategy for playing the Star Wars Battlefront Companion App:
Defeating enemies without losing any of your own units, increases the score multiplier (up to a max of x3). Therefore, never enter a battle that you are likely to lose.
Defeat lower scoring enemies first so that you have higher score multipliers for the higher scoring enemies.
Defeating enemies earns you ranks. You get a proportionally higher score the more ranks you get (rank 1 gets you 50 points, but rank 2 will get you x3 as many points)
Weaken units with left hand cards, but try to defeat them with right hand unit cards, so you gain the ranks.
Losing a unit is worse (score wise) than losing shields or some base health.
Some Sieges start with a Base Health of less than 10 and not all Sieges have health rejuvenate over time. If you are on a Siege level like this, make sure you add the 'Increase Base Health' card, since Base Health impacts score bonuses.