Your HP bar problem could be solved by grouping with a priest, but it sounds like you're not interested in playing with other people! Oh well!
Already knew that, but it still doesnt matter if you take large amounts of damage which is what happens against bosses on occasion. To make the healing work against bosses you would need to basically have the caster do the healing circle, have everyone else heal first then have the shield sage heal last due to them pulling the monster with them. Otherwise you are just dragging the boss into your group.
Against things like the cyclops for example you would be more busy healing those who are trying to "drag it down" by clinging onto it and hitting triangle. As they will be taking generally more damage from random hits with it thrashing around than the Shield Sage. The Shield Sage usually will only take damage from the grab attack which cant be blocked or if they are not paying attention to facing / stamina. Not too big of a fan of the enraged = enemy stamina bar that needs to be depleted to down them. As even with a party of humans you are taking unnecessary damage trying to drag that stamina bar down. You can chip away at it with normal attacks but its a very small amount. The upside to letting yourself get grabbed though is can have the healer create a healing circle near where you will land after getting released also your allies are pulling at the boss to drain its stamina. Depending on the areas where you are fighting too its possible to cheese bosses also by getting them stuck on areas with height differences.
Have not bothered with the other boss types out there as tested it out a few times and take way too much damage from normal attacks making it too much of a risk. Fairly decently equipped also, so its more a level issue than anything granted we are about the same lv as the thing but as with any MMO never take a bosses level at face value. The sphinx for example does some insane damage with its normal attacks, so have been trying to avoid contact with those. They will start appearing once you raise the level for an area by doing quests and whatnots raising your fame then talking to the area master that is usually in the inn. That is also how you get new dungeons to unlock also by reaching the various tiers for the specific area. All that info can be found out from the area master.
I stay out for hours at a time so work the HP bars pretty low before returning. Most of the time against normal enemies its not too much of an issue with a balanced party of pawns.