If you are interested in earning $300 Million + to spend in Grand Theft Auto V and Grand Theft Auto Online, here we are with a very useful guide that will help in unlocking this achievement. Follow it step by step and you will gain that amount of in-game credits very, very soon:
1: Begin the story and go through all the missions until the LifeInvader assassination. Execute that assassination too, as it won’t change the amount of credits you can gain at this point
2: Things do more serious once you get to The Hotel Assassination. Here you have to damage Bilkington in order to raise Betta Pharmaceuticals' shares. Once you do this, Betta Pharmaceuticals' shares will peak at around 80%, so be careful in that.
INVEST BEFORE MISSION: [BET] BettaPharmaceuticals (BAWSAQ)
SELL AFTER MISSION: [BET] BettaPharmaceuticals (BAWSAQ)
You can buy shares from Bilkington too, but it will take a longer period of time before you can sell those at a good price. Anyway, both the operation won't get you to have a huge amount of in-game credits at this point.
3: Finish the game completing the storyline. You should have by now around $40 Million to invest on each character, and from now you can proceed investing some real money on shares.
4: In The Multi Target Assassination, your quest is to damage Redwood in order to raise the shares of Debonaire. Redwood’s shares will grant you a gain of around 80%, while Redwood's peaking at 300% or so after a couple days.
INVEST BEFORE MISSIONS: [DEB] Debonaire
SELL AFTER MISSION: [DEB] Debonaire (LCN)
BUY AFTER MISSION: [RWC] Redwood
SELL AFTER REBOUND: [RWC] Redwood
5 – The Vice Assassination will make you harm Facade's shares so you can have a better gain thanks to Fruit. Fruit shares will peak at around 50%, while Facade’s at 30% or so.
INVEST BEFORE MISSION: [FRT] Fruit (BAWSAQ)
SELL AFTER MISSION: [FRT] Fruit (BAWSAQ)
BUY AFTER MISSION: [FAC] Facade (BAWSAQ)
SELL AFTER REBOUND: [FAC] Facade (BAWSAQ)
6 – In The Bus Assassination, you only have to damage Vapid stocks and wait for them to rebound; once you do this, they will peak at 100%.
BUY AFTER MISSION: [VAP] Vapid
SELL AFTER REBOUND: [VAP] Vapid (BAWSAQ)
Go have some sleep and wait for around 8am, so you can check if the operation went ok or not.
7 – The Construction Assassination doesn't have rebound, you only have to get GoldCoasts business back from another construction company. GoldCoast stockes will peak at 80% or so.
INVEST BEFORE MISSION: [GCD] GoldCoast
SELL AFTER MISSION: [GCD] GoldCoast
There's an alternative way you can proceed through, anyway. In order to use this way, you have to have the game finished, Lester's missions untouched (apart from the first mandatory one), and maximum amoun of money ($35 Million) from the big one.