Resident Evil 6

Professional difficulty is unlocked from the start. Difficulty achievements stack so you can complete the game on pro and unlock all the difficulty achievements below it. You can check and see the highest difficulty you’ve completed a chapter on from the Chapter Select or Records menu.

You can also change difficulty between chapters from the Chapter Select menu. Ie. If you’re playing on normal difficulty and partway through the game you’ve bought enough upgrades and feel like you can handle pro, you can go to chapter select, choose the next chapter and choose pro difficulty. Then after you've finished the game, you can go back and finish the previous chapters on pro difficulty.

Choose pro difficulty from the beginning to save time if you’re confident in your abilities. Otherwise, run through the game quickly on amateur difficulty to learn the ins and outs of the campaign, and also unlock infinite ammo which will make your professional playthrough much easier.

On professional difficulty, QTEs have shorter timers, you take more damage, and enemies have more health. Look for breakable objects to collect ammo and healing items. Buying skills with the skill points you save will be very beneficial. You can have up to 3 skills equipped at one time, but after completing 1 campaign, you can have up to 8 different load outs. You can change loadouts from the  menu during the game and switch to whatever the situation calls for.

Some of the more useful skills I found are:

Firearm - Increases damage done with guns (up to 50%). This will help you kill enemies faster, and save precious ammo.

Defense - Decreases damage done to you (up to 50%). This will help you survive longer, and allow you to save your health tablets for when you have bigger fish to fry.

Field Medic - This is an incredibly useful skill if you're playing solo. Whenever you run out of health and your partner rescues you, they'll also heal a few squares of your health. Normally after being rescued, you stay in a near death state until you recover a square of health. With Field Medic, you always recover with a few squares of health, without having to use your own tablets. You'll almost never die since no matter how many times you lose all your health and get downed, your partner will heal you over and over.

Infinite Ammo (Unlocked after completing all campaigns) - This is an obvious choice. Not having to worry about your ammo supply is a huge advantage. Combined with Firearm Lv. 3, Professional difficulty becomes a walk in the park. Cost for each type of infinite ammo ranges from 79k-99k. Since you might not have an abundant supply of skill points after just one playthrough, your first choice should be something that's used by almost every character, such as assault rifle, shotgun, or pistols.

Breakout - Useful for if you're having trouble with QTEs.