I'm just posting this for the people who are too lazy to read the wiki.
This is not a insult to people who do know.
Edit: And for the people who keep saying: "normal defense is useless in t3.. y u want that?"

CAUSES OF NORMAL DAMAGE IN TIER 3

1. Trojans. All Trojans do pure normal damage, in all tiers.
2. Lumbers. All Lumbers do half normal damage & half elemental damage in Tier 2 & 3.
3. Devilites. All devilites do half normal damage & half shadow damage in Tier 2 & 3.
4. Gremlins. All gremlins (Except flamethrowers, they do elemental, & menders, who do none.) do pure normal damage in all Tiers.
5. Retrode melee. All retrode melee attacks do pure normal damage in all Tiers.
6. Blast Cubes. Blast cubes, super blast cubes, and ultra blast cubes all do pure normal damage in all Tiers.
7. Mini Jelly's. All mini jelly's (including green) do pure normal damage in all Tiers. (Danger rooms in T3)
8. Rock Jelly Cubes. All rock jelly's attacks do pure normal damage in all Tiers.
9. Imposto Cubes. All imposto cubes attacks do pure normal damage in all Tiers.
10. Lichens. All lichens tackling attempts do pure normal damage in all Tiers.
11. Mecha Knights. Mecha Knights do pure normal damage in all Tiers.
Note: Status mecha knights do half elemental half normal in Tier 2 & 3.
12. All scuttlebots attacks do pure normal damage in all Tiers.
13. Vanaduke. All vanaduke's attacks do pure normal damage.
14. Kats. All kat bites do half normal damage & half shadow damage in Tier 2 & 3.
15. Slag Guards. All slag guard attacks do pure normal damage.
16. Zombies. All zombie attacks do half normal damage & half shadow damage in Tier 2 & 3.
17. Swarm Turrets. All Swarm Turret attacks do pure normal damage.
18. Brambles. All Brambles do pure normal damage.
19. Spiked Wheels. All Spiked Wheels do pure normal damage.
20. Explosive Boxes. All explosive boxes do half normal & half elemental damage in Tier 2 & 3.
21. Spiked Jellies. All spiked jelly tackles do half normal damage & half piercing damage in Tier 2 & 3.
(All credit goes to the Wiki, though I typed this myself.)

Quite a few more then you thought, eh?

Cheers,

~W