Generally speaking, the Sanctuary Guardians aren't too difficult if you really throw everything you've got at them (with the exception of the Carbon/Diamond Dog, that was a close call...).

Edit 12/27/18: Do keep in mind that this is my first playthrough of the game, so some things I do might not be very strategic.

Time Stamps:

0:00 - 1st Sanctuary Guardian - Titanic Ant
1:32 - 2nd Sanctuary Guardian - Mondo Mole
3:11 - 3rd Sanctuary Guardian - Trillionage Sprout
7:02 - 4th Sanctuary Guardian - Shrooom!
9:37 - 5th Sanctuary Guardian - Plague Rat of Doom
12:27 - 6th Sanctuary Guardian - Thunder and Storm
15:40 - 7th Sanctuary Guardian - Electro Specter
21:27 - 8th Sanctuary Guardian - Carbon/Diamond Dog