I've had a few people DM me about this achievement, so I thought I would post a solution here. I want to give credit to Claudio 10 Juve for this - his advice was valuable in creating this solution.
A CO-OP Playing Style in Pro Evolution Soccer 2019 is earned during a CO-OP match, when playing with another person. These can be earned in either a match against other online users or the COM. For the purpose of this achievement, we will be playing against the COM.
The easiest Play Style to unlock is Bomber
. This is awarded for being prolific in front of goal. To unlock this, do as follows:
1. Create a CO-OP lobby, choose 3 vs COM, pick a strong team (Man Blue) against a lower rated team (I used SW Red), and ensure the difficulty is on Beginner.
2. Invite your friend into the lobby and start the match.
3. Play the game. Now, whoever is unlocking the achievement should be SCORING at least 7-10 goals in the game, and MOST (if not all) of these should come from your partner's assists. Try to score from inside the penalty area as often as you can during the game. Resist the urge to score from a solo run, or shoot from range - I found that this counteracted and slowed the progress on the achievement.
Doing this, you should get the Bomber Play Style after 1-2 games. It could take 3-5 games though, so don't fret if it does. I did this in three different boost sessions, and none resulted in the same outcome.
Also, if you have played a few CO-OP games before attempting this, it might unlock quicker. Once unlocked, simply swap with your partner (if he needs the achievement too), and repeat.