There are 5 training missions and you can get up to 3 stars in each though the achievement only wants you to earn 1 on each mission which will be easy as the first star on all 5 is for simply completing the mission and since they are training they are simply to complete.
The training missions are within the Missions menu and are:
• Beggar's Canyon - Flying mission with an X-wing taking out mostly Tie Fighters.
• Endor Chase - Short mission with you on a speeder bike taking out rebels. (Can drive at half speed and not even kill the rebels)
• Overpower - You're in an AT-ST escorting an AT-AT
• The Dark Side - You choose to play as either Vader or The Emperor and storm the rebel base.
• Invasion - You fly a snowspeeder taking out AT-STs and AT-ATs. Hardest one to do, I suggest taking out the AT-STs first then go for the AT-ATs. Make sure you fly away fast and turn around and go slow to shoot the AT-STs, aim using
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You will also earn the following for completing one mission