Flag Gems
You can change your squad's flag and earn Gems. Each flag will have their own requirements.
Easy Gems
If you click the flag in the upper corner you can earn Gems easily completing the said task for each flag that needs to be unlocked.
Repeat Stages
if you repeat the stages 5 times in a chapter you will be rewarded additional Gold and 1 Gem. The easiest way to do this is if you grind in the easiest level.
Get Better Eqipment
Always look to use the Duel Medals you earned on getting better equipment as this will enable you to move to the next stage easier.
Equipment Fusion
To fuse two pieces of equipment together they must have the same stars and same +9. The rarity of the result is random.
Khan
Khan damage increases by 100% when one of his allies has fallen so a good tactic is to not include him as one of your 1st three fighters so when an ally falls and Khan enters the battle he is already buffed with the 100% damage increase.
Sudden Death
Deadly and fast projectile bullets will appear in order to end the game quickly once you have run out of time in Adventure mode, this is known as Sudden death.
Collect Rewards
Don't forget to collect your reward once you have cleared out every stage with 3 stars. The red ring on the screenshot below indicates what needs to be press to get the reward.