Here is a Guide for how to obtain cores so people can stop asking.

STEP 1

Fight any robot/mecha/boss and kill them with their weaknesses

STEP 2

You will obtain a Shard of the robot/mecha/boss’ Core Type. Collect 3 More to obtain a Core

STEP 2.1

It's not guaranteed you will get a Core straight up, so you can just get the Core Immediately by 50%

STEP 3

After Obtaining 4 Shards of a Core Type, you get a Core of the corresponding shard type!

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GUIDE FOR CORES

Neutral - Kill Met’s, Roll, Disco, or Rhythm
Cutter - Kill Picket Man’s, Pierro Gear’s, Needle Ned’s, Cut Man, Metal Man, Needle Man, Yamato Man, Blade Man, or Punk
Impact - Kill Flea’s, Monking R’s, Ribbitron’s, Guts Man, Hard Man, Knight Man, Spring Man, or Strike Man
Freeze - Kill Peng's, Nitron B's, Ice Man, Blizzard Man, Freeze Man, Frost Man, Cold Man, Chill Man, or Tundra Man
Explode - Kill Bombomb's, Killer Bullet's, Bomb Man, Crash Man, Napalm Man, Burst Man, Grenade Man, Pirate Man, Blast Man, or Ballade
Flame - Kill Tackle Fire's, Telly's, Nitron R's, Pyre Fly's, Fire Man, Heat Man, Pharaoh Man, Flame Man, Sword Man, Burner Man, Magma Man, or Solar Man
Electric - Kill Spine's, Nitron Y's, Elec'n's, Bulb Blaster's, Elec Man, Spark Man, Gravity Man, Cloud Man, Clown Man, Dynamo Man, Plug Man, Sheep Man, or Fuse Man

Will be Updated Soon