The fastest way: spend real money to buy Helix Credits and which them the money Packs. Other way with real money, buy infinite booster for Money earn and XP Boost, so you get more on every Mission, etc. But to spend real money is always something else. An other fast way: Take every mission, and look which are close together, so if you there you can make not just one mission. AND work as mercenary, so help Sparta to Beat Athen and Athen to Beat Sparta, this gives XP and money. Plunder your fallen enemies, especially better enemies and mercenaries have money and or good loot which you can also sell. The state treasury is always good amount of money and is one possible Option to destabilize an Region. I hope this answer helps you, even When my english is Not the Best.
Honestly, buying the booster packs