It might sound a bit worrisome, but it’s simple. Pouring all of your XP (and QP) into one CE is a waste. It’s better to upgrade multiple low rarity cards, and use those to upgrade the CE of your choice. It’s simple, but there’s a couple of methods you can choose from. Some things to keep in mind before beginning:
This method is used to level up ONLY Max Limit Broken (MLB) CEs. Usually CEs that give attack are better than ones that give HP. There are exceptions. This is a time-consuming process meant to be used in the late game. This can save you potentially hundreds of millions of QP in the long run. FP is easy to get, but you’ll need a LOT of it. You’ll probably want a show or something else to watch in the background.
Method 1: Most QP-Efficient
This method will save you the most QP. At the cost of your time. And potentially your sanity. You’ll start off by MLB-ing any 1* CE. This is done by upgrading it with four other copies of the same CE. This will be your “bomb.” Next, pick any 2* CE. Enhance it, but make sure you don’t go above 18,000 EXP, or a super/great success will waste EXP. Repeat this process as many times as you want. After this, select any 3* CE. Make sure it’s one that you won’t miss later. Take the 2* CEs from the previous step and use them to upgrade it. Try to make sure it doesn’t go past level 18, or you may waste EXP. Now, take those 3* CEs and use them to upgrade the 1* CE that you MLB’d at the very beginning. Do this until it reaches the max level (in this case, 50). Once you’re done, congratulations! You’ve created your first “bomb.” You’ll use this to upgrade a MLB’d CE that you actually want to use. This method will save you the most QP possible, but at the cost of time. Only use it when you need every bit of QP possible, and have plenty of spare time.
Method 2: Most Time-Efficient
This is typically the method that I (and most other players) use. It costs more QP, but doesn’t take as much time. You’ll start off the same as the previous method. MLB any 1* CE. Like before, this will be your “bomb.” Now, instead of upgrading 2* CEs, just dump everything into that 1* CE. That’s it. No other steps. Just keep upgrading until it reaches level 50. Congratulations! You’ve created another “bomb.” The last bit is simple. Feed it into the MLB CE of your choice. In my case, I’m leveling up the Imperial Capital Grail War CE. As you can see, not only does one unfinished CE bomb yield more EXP, it also costs significantly less QP. Note: 1* CEs are used as the base because upgrading them costs less QP.