[1.12.2]How to make and test coremods using IDEA?

[OpenSourceForce]

I want the creepers to:
1. They gathered and went in groups
2. They were able to break 1 obsidian block, but die at the same time. Only one of the creepers in the group simultaneously broke the same block
3.After the death of the player, the remaining in the group creepers broke the player's base

(sorry for machine translate)

I think, I need coremod to do it. But I can't find how to test coremods in Intellij IDEA. I read cluegooner's tutorial but I think it's outdated because it didn't work for me. Please help.

Also please say how can I use this library because I'm completely unfamiliar with asm library and java bytecode.

Edited November 1, 2017 by OpenSourceForce