THE THEORY
In the Level 3 version, it was easy to get your health to around 10-19 for use with protective spirit (10% damage cap brings you to -1 per hit). As you level up, keeping the ideal health range becomes a challenge, but you can attain it by juggling superior & major runes, vigor runes, and +health upgrades. As you approach level 18 and beyond, you are no longer able to use the same method, as the damage you are taking (per hit) becomes too great to simply health through (when facing marge mobs).
The answer is Protective Bond. As it reads, you cannot take more than 5% of your health in damage at once. You also lose energy with each hit (-2 energy at maxed/16 Protection you may notice). This presents two new problems:
1) How to effectively overcome the energy issue?
Losing 2 energy per hit, you may think to add in Balth'z Spirit and Essence Bond (as described in my level 3 post) for a +2 energy per hit. This provides a break even point... but no extra energy. While you may try to use bonetti's for energy regain, there is a much easier solution: Find a Protective Icon with +1 to Protection Prayers (20% of the time).
The +1 Protective Icon can be found in several places - but the easiest appears to be just outside Bloodstone Fen (map there, and step back outside). A collector outside collects Jungle Troll Tusks (conveniently there are a few packs nearby) in exchange for the icon.
Now when you cast Protective Bond, make sure your Protection is maxed (16), have the icon equipped, and cast/recast your bond until you see -1 energy in the description (wave the mouse over it in the top left to verify). If you cast a -2, simply double click the icon (by default, over your skillbar) to cancel and retry.
Once you have it at -1, you may redistribute attribute points away from Protection for better use elsewhere (the stats for your bond will not change once cast). Now when you are hit, you lose 1 energy from the bond, but gain 2 from your 2 energy enchants, for a total of +1 energy every hit. For this reason, your build will be MORE effective the more you are mobbed (it becomes painfully slow to kill one or two creatures with so little energy regen).
If you want a little more energy, grab a +5 energy sword at Henge of Denravi. Remember you don't need any points in swordsmanship to get the energy bonus, so it's a free +5.
2) How to lower that damage (per hit)?
At default, with 5 superior runes (yes, one will be a repeat), you will sit at 105 health. With protective bond, the max damage you can take per hit is 5 (105 * 0.05 = 5.25). With a maxed mending, achieved with 14 healing prayers (+4 regen = +8 health per sec), that is a whopping 1 hit per second you can take and mantain full health. Healing Breeze when used with Mending should cap you at +10 regen (+20 health per second) for a total of 4 hits per second (4 x 5dmg = 20 dmg). Since the fastest most monsters can attack is 1.33 seconds, and much slower for some... you can take roughly between 5 and 9 monsters safely. This is fine for some people and they've been doing it just like that.
However, there is a more elegant answer: the mythical -50 health focus item. Since energy is no issue now, losing a "real" energy focus is of no consequence. This -50 health item can be found at Symon the Scribe in Ascalon City as a reward for the Cities of Ascalon quest. Either start a new character to get one, buy one off another player, or whatever.
With the use of this item, your health can now be brought down to 55. 5% of 55 is 2.75, so 2 is the max damage per hit you can take. Much better than 5. This means with only mending, you can take 4 hits per second. With mending + breeze, you can now take 10. As previously stated, monsters attack slower than once a second, so you can effectively tackle significant mobs with this build.