ReCore

To earn this achievement you must level Joule up to level 30. You need to reach level 28 to access the final area anyway, so levelling up quickly is a must. The level requirement has been removed on the Definitive version of the game

A great method to increase your XP gain is to utilise the equipment bonuses of your corebots

For Duncan equip the White Hammer Fist AP-3 set which gives a 15% Team XP GAIN.

On Seth Equip the Orange Iron Tusk SP-DR set. This increases combo duration by 40%. Combos play a huge part in gaining xp so this set will really help you earn high xp scores from your battles. (A full set must be equipped to gain the bonus, not parts of the set)

As well as replaying dungeons I found that replaying the fourth and fifth floors of Eden Tower gave good quantities of XP.

Rather than destroying enemies, attempt to extract their cores whenever possible particularly of higher level foes. If you find yourself surrounded by enemies make sure that you use both your lethals as well as your corebots, this will stagger the enemies, do serious damage and preserve your combo. Extracting a core will restore part of your health as well as preserve your combo.

One of the best tools at your disposal for levelling up is also the Instant Extraction. You need to use this carefully and selectively. Don't simply use it on any enemy. Try and build up your combo as high as possible and save the instant extraction for a large rare creature. You earn an instant whenever you reach a combo of 10, even if you then lose the combo you will have that Instant stored up. Once you use this it will end your combo but will double the XP you recieve. So using this wisely can net you huge XP gains. Again corebot equipment which extends combo duration is invaluable for this.

Replay dungeons, but check the yellow level up bar before and after to see how much XP you are netting per run. Higher level dungeons will net you more XP but if you constantly die and struggle then it may be worth playing a lower level dungeon.

Using the combo/ Instant Extraction system wisely makes the difference between earning 100 XP after a ten enemy fight or 1000 XP. If you go into each battle just spamming the shoot button it will take you a long time to reach level 30.