This is from Deities & Demi-Gods page 56
Deities do not have challenge ratings.Entities of this sort are so far above the realm of mortal heroes that determining their level of power in re
Relation to mortals becomes almost meaningless.
After that it also goes on to say...
If you feel the need to calculate a Challange Rating for a deity,try adding its total Hit Dice to its divine rank. The result may or may not be an accurate estimate of what level of characters could Challange this god...
Also for your information a divine rank of 0 is quasi- deity or hero-deities.
A "Demi-God" is divine ranks 1-5 this information is found on page 25 of Deities and Demigods.
So with that said your "Quasi-Deity's" CR would equal the amount of hit dice he or she would have. So if your Quasi-Deity had 20 outsider hit dice as the norm of any deity you would add 20 to the number of class hit dice he would have easily putting him out of reach with a level 18 party.
But as you stated in your question your creature only has 20 hit dice so his "CR" would be 20. No adjustment for the divine rank of 0.
Duskblade 13/Crusader 1/Jade Phoenix Mage 6 seems to me to be much superior to Duskblade 20. You hit the really big duskblade class feature and retain your highest-level spells.
The issue is timing. The best levels to take Crusader are 5th, 9th, or 13th. Coincidentally, however, 5th, 9th, and 13th are also the levels where it is most painful to have a single level of crusader rather than being a single-classed duskblade, as those are the levels at which you get new levels of spells (and full-attack arcane channeling, in the case of 13th). You have two competing goals: get the next spell level (and full-attack arcane channeling) as soon as possible, while having maneuvers for as long as possible, since they are useful.
Therefore, I recommend taking the crusader level at 6th, 10th, or 14th; that is, I’d take the next spell level first, rather than the maneuvers. Here’s a breakdown of each option:
Duskblade 5/Crusader 1
You can take 2nd-level maneuvers with your first five maneuvers. This means, primarily, that you will have mountain hammer for the longest possible time, which is awesome. Other than that, though, you’ll probably still take a number of 1st-level maneuvers.
Duskblade 9/Crusader 1
You can take 3rd-level maneuvers and stances with your initial set; you can skip crusader’s strike for revitalizing strike, you can still take mountain hammer, and the amazing white raven tactics is available to you. Thicket of blades opens up as an interesting stance option; martial spirit is pretty small at these levels, and thicket of blades greatly improves your presence on the battlefield.
Duskblade 13/Crusader 1
Clearly if you haven’t taken crusader by this point, you should. Duskblade has little to offer, so you lose almost nothing at this point. The disadvantage of waiting this long is that you haven’t had maneuvers at all until this point. The advantage, on top of duskblade features ASAP, is divine surge, an excellent offensive maneuver.
The Awkward Bit
OK, so now that we’ve covered how to do it, there’s one other thing to consider: you cannot use arcane channeling and a martial strike at the same time. Arcane channeling requires a standard action attack or a full-attack, so the attack(s) you get from a strike don’t count. This makes strikes massively less useful to you than they would be to others. A boost or counter heavy selection of maneuvers solves this issue; you could do pretty well with defensive rebuke, shield block, and white raven tactics. But missing out on fantastic options like mountain hammer, tactical strike, the various healing strikes, and divine surge is a disappointment.
It also means you don’t have especially great mobility. You have invested 13 levels in duskblade to allow you to full-attack with spells, so you really do want to be making full-attacks. Crusader doesn’t help with that. Swordsage or warblade would help a little, since Tiger Claw is pretty good at that (sudden leap, arguably pouncing charge), but cleric for Travel Devotion or barbarian for Lion Spirit Totem (Pounce) might be better. Those options don’t lead into a great prestige class like jade phoenix mage, but they still might be better options.
The Other Thing
Finally, item-based healing is generally sufficient in 3.5. In-combat healing is rarely an optimal strategy, barring emergencies; the crusader is far better at this than other classes, but you shouldn’t usually need it. The aura you describe all-but-eliminates the worst emergencies, too. Wands and healing belts are quite sufficient for out-of-combat healing. As Jeor Mattan mentioned in comments, the real draw of actually having a cleric is the various options he or she would provide for removing status conditions, since items have HP-healing pretty much covered (and until you get heal, clerics’ HP-healing spells are pretty poor), and crusaders don’t cover that at all.
This is another situation where a level of cleric may be more useful to you: it would allow you to use wands of any cleric spells you like, with no Use Magic Device check. That will cover a lot your needs.
Best Answer
Use the spells delay death and beastland ferocity
The 4th-level Clr spell delay death [necro] (Spell Compendium 63) says
The spell targets 1 creature within close range as a standard action and has a duration of 1 round/level.
The 1st-level Drd spell beastland ferocity [ench] (Spell Compendium 25) says
The spell targets 1 touched creature as a standard action and has a duration of 1 minute/level.
Both are low enough level to fit in wands, and some prestige classes can even cram them into potions.
A legalistic DM may kill the creature affected by both spells when the creature hits –10 hit points anyway, as per the spell beastland ferocity, but the effects of the spell delay death should alter what would otherwise be dying normally for the affected creature. Nonetheless, before whipping out this strategy at the table, I strongly urge first clearing it with the DM.