Spirit tree is needed 100000%.
Guardian spirit and Strenght of spirits is what you want from the Spirit tree.
You might as well pick up Peaceful Aura as well, and since you must pick Barrier, you might as well improve it.
You need 5 points in spirit tree, 6 with improved barrier, thats it.
You only need a few points in knight enchanter, 5 points to be exact. The rest is not needed at all, unless you want to deflect missiles, which is handy.
All in all, you need a minimum of 10 points to be a full blown KE that can steam roll nightmare.
If you truly want to shine, you add in another 2 points for a whopping 12 points for a full on final build.
And those 2 are for the upgrades of Barrier and Spirit blade.
Barrier caps at 100%, there is no way to go past that.
Guard can go past that with passives.
However barrier will have from the passives I listed: 50% stronger (basically you have 150% barrier with strenght of spirit), you also degenerate shields 35% slower with Knight protector in KE tree, and mana is never a problem due to Combat Clarity in the KE tree.
Again, Guard can surpass 100% with talents. You can combine both the Vanguard+Champion talennt trees for maximum armor and guard.
Which is why Blackwell takes over as hyper tank without competition once Specs are unlocked. Making a perfect partner through the campaign for your Spirit blade spamming 10 to 12 point max needed to destroy Nightmare, mage class.
Enjoyz the game:)
PS: If you truly want to max him out you pick
Improved Fade step and Winter stillness from Winter tree.
And improved Fade Cloak from KE tree. So thats a total of 5 points more you need to invest. But now you are invincible.
The rest of the skill points you might as well stick into anything that can give you cool down reduction, such as "Clean Burn" in Inferno tree.
This synergizes extremly well with the KE due to your spammability of spells and Clean burn have no internal cool down before you reduce abilities by 1 second. It also comes at a meezy cost of 3 points into Inferno.
So all in all, 19 points for a, la carte