To hit flying enemies simply hit them when they descend to a level that you can do so.
This occurs at two moments, the first moment is when they are flying towards you and the second moment is when they change direction.
Hope this helps :)
The easiest way to get 3 stars for every stage is to do it right the first time as you get 400 extra points for winning the first time, in addition you should also be racking up every single kill with a lot of kill bonuses
If you play an archer a way to attack enemies further away from you is to go a distance away from the enemy and fire in the general direction while holding shift + left click, it does not matter if the enemy is off screen, the arrows would still hit and damage them if it hits
Hope this helps :)
If you find the screen too small just zoom in with pressing ctrl and + at the same time. Besides using + you can also use the mouse wheel to change the zoom level.
After zooming in, you can press F11 for full screen so you can zoom in much more and be able to see the entire game fully without any obstruction by the task bar, favourites bar etc.
Reinforcement points refers to the points you use to summon friendly units into the battlefield. Guard points refers to the defence of a unit,which affects its ability to take damage (enhancing its survivability). Recover time refers to the time it takes for an injured unit to heal itself over time. Respawn time refers to the time it takes for a dead unit to revive itself. Aim time refers to the lag time it takes for a unit to target enemies within range. Reload time refers to the time taken to reload to a fresh clip after discharging the ammunition of a clip completely. Sight distance refers to the range it takes for a unit to be able to fire upon an enemy unit. Projectile AoE refers to the area of splash damage whereby enemies within the zone would take damage. Armour piercing refers to the ability to deal increased damage to enemy armour (and other vehicles). The 3 star upgrades for the attack helicopter and the bomber increases the overall area being targeted.