Do not fight Aggressive aliens because needs two shoots and you loose CP and that means less power
Fight Arena is dedicated for players organised in team
So, to know when you will be able to beat each level different, alien, you should use Damage Calculator:
Beginners players (level 7 or less) are welcome to fight in the Training Battles.
Battles are fought on the actual map of the attacked city. To fight in a battle players have to be in the City where the battle is fought.
...you can take part in RAID (anytime you want, can join with us...), you got 45 xp and that is helping you to climb levels faster, also you got Boosters, that can use it when you travel by car, to makes your trip faster and 10 Blue and Red pills, that can transformed into stamina,