One Better Achievement in Rocket League
Increase the level of a Certified Item
How to unlock One Better
This is another luck-based achievement. You'll want to play online games (regular or competitive) in hopes to get a "Certified" item drop at the end of a match. The “Certified” attribute makes that item track a specific stat while it is equipped in online matches. As you earn more stats with your item it will level up.
Once you equip the item, start playing more online matches and as you play the item will track your stats (goals, saves, assists) and over time will level up the item. Aim to complete the task that will level up your certified item and you will eventually level it up and unlock the achievement. Depending on the specific stat that is tracked you could need to get anywhere from ten to fifty of those tasks. When an item levels up, the achievement tracker will go up 33% each time.