best tip in the game is learn support and height v range...
support equals you live longer in a fight and do more damage
on your own almost means death... unless you are a higher level type unit or have good defence v attacker
height adds bonus to shots so do more damage
cliffs or hills etc
range takes away damage most of the time, so better to not do really long shots
some units can fire at longer range, but nearly all do more damage or better chance of a good hit the closer the range and skill
and finally cover, tree's and other types of terrain give bonus to cover, so use it to your advantage
any unit in cover take little or no damage in cover, some of the time but not always, depends on attacker and type of attack
master some and you will find it easier, master them all, and you will finish all 20 missions

oh and welcome to Hell...