1) Can I see the human player?
2) Do I want to move towards him, stay put, or back off?
3) Is my turret aiming at him? Should I rotate it?
4) Do I have a clear shot?
5) Which way is the human moving? Where should I aim to lead the target?
6) Am I ready to take a shot?
7) If I don't see the human player, which way should I go?
8) Is there something blocking my path? Which way should I turn instead?
9) Is a tank shell coming at me? Where can I move to avoid it?
Just a small example of all the decisions your AI would have to consider. After you've got these all listed, you need to come up with how your AI will come up with the answers. For example:
1) Can I see the human player?
a) If he's closer than 500 units and I have line of sight, I can see him.
b) If I have seen him in the last 10 seconds and he's no longer in sight, remember where he was at.
2) Do I want to move towards him, stay put, or back off?
a) If his back is turned to me, I will move to him
b) If he's moving closer to me, stay put
c) If my health is too low, back off
3) Is my turret aiming at him? Should I rotate it?
a) Rotate my turret to face him or his last known position at all times
4) Do I have a clear shot?
a) Is there something between me and where I want to shoot?
b) If there is, I should move somewhere else
5) Which way is the human moving? Where should I aim to lead the target?
a) Based on speed and direction of the human, and speed of my bullet, calculate the trajectory I need to fire at
b) Add/subtract some degree of error to this trajectory based on difficulty level of the AI
6) Am I ready to take a shot?
a) I can only fire one shot every 5 seconds, make it count
7) If I don't see the human player, which way should I go?
a) Can I go straight? If so, move forward
b) At random intervals, rotate the tank if the path is free to go in that direction
c) If I am stuck, determine which way I can move and rotate the tank in that direction
d) If my health is low, make sure there are obstacles between the human and me
8) Is there something blocking my path? Which way should I turn instead?
a) If I don't see the human, turn a random direction and try that instead
b) If I see the human, choose a new path to a clear shot
9) Is a tank shell coming at me? Where can I move to avoid it?
a) Determine the angle of the attack and move perpendicular to it if free to do so.
Then, once you have your decisions and how your AI will handle them, then it's up to you to take your flowchart and code it. By this, it means you have to come up with the programming for "If I see the human, choose a new path to a clear shot" and what that means for your game. This could be several lines to hundreds of lines of code for each part, depending on how complex your AI is.
Then, to go a step further, you can provide the AI some learning capacity:
1) Does the human player tend to back off when he's hurt? If so, how should I react to that?
2) Does the human come straight for me instead of flanking? If so, how should I react to that?
3) Is the human fairly accurate with his shots? If so, should I use cover more?
Again, how to code these measurements and responses is up to you, but you should already be able to see the value in thinking about all these details and writing them down ahead of time, so the coding will be easier when you're ready to program the AI rules of engagement.