As in, how much defense do I have to have before attacks that would be damn near lethal become nothing more than a flesh wound, losing maybe 2 or 3 pips at the most instead of 8 or 9 from a single blow?
How much defense do I need to achieve this, because so far I've seen no meaningful mitigation from my defensive values.
Krakob, you mean Grey Feather? Because Grey Owlite is the shield, not the armor.
I hate how t3 becomes almost entirely oriented around offense (and status resistance).
^best defense is the best offense!
Don't bother with defense on tier 3, or anywhere else regardless... The damage defense scale on armor is pretty bad. If you insist on it though, be sure to grab plate armor and appropriated perk, UVs and trinkets. Maybe that'll reduce at least 33% of the 12 pips of damage the enemies can do.
The best defense is health boosts and a decent internet connection.
Actual defense as a stat value shouldn't be too big of a worry. I primarily wear defensive armors and have a high amount of sparks of life to prove my survival ability, However...
Spiral Knights is pretty much backwards in the roles of offense and defense. Defense is more measurable to skill rather than gear, so people will often follow the offensive gear mindset while still retaining a large amount of defense. You'll see people complaining about difficulty when defense actually becomes an important factor (March of the Tortodrones, Grinchlin Assault, certain danger missions or shadow lairs), but just about all other content in the game is dependent on offense.
So if you want to improve your survival, look into health increases via trinkets or sprites.
The effect of defence diminishes as it passes half the total damage it's defending against. Therefore, you cannot hope to reduce incoming damage by much more than 50%-70%.
A strong attack in D24 take something like 15-20 pips if I recall correctly.
Yes, defence is quite weak. If you wanna get anywhere with it, Drakon's Flame Barrier is a must. If you're looking for normal defence, you can get 301 from Ancient Plate and 201 from Volcanic or Ironmight on Depth 25 (values for other depths are unknown). Bear in mind, these do not have good specialised defence. If you're looking for that, the most powerful sets are of course Royal Jelly/Ice Queen, Dread Skelly, and Grey Feather. The sets with split specialised defence have decent amounts, too. That said, defence is generally pretyt worthless in this game unless you're investing in it wholeheartedly.
Lastly, I'd like to point out that poison weakens the damage your enemies inflict. If you want something really interesting, use Obsidian Crusher in combination with Scorching Barrier and you'll see what's more or less the only way to tank in this game. The damage should be decent as well.
EDIT: I forgot to point out, you should most certainly read the wiki page on defence if you haven't done so yet.