RD Playguide: Endgame 3
Base Prep:
Health-regenerating skills are redundant on Kurth and Ena this time, so consider swapping those out and passing them on to other units.
Parity is a big plus here on your Marksman, since you can negate tile bonuses for the Dragons while still attacking with impunity.
Dragonfoe + Arbalest is a great combination, and is even better against White Dragons.
Anyone who may strike Dheginsea needs Parity or Nihil. You do not want him getting Ire. Slower and/or more vulnerable units need not even make the attempt.
Adept, Vantage, Resolve, and Cancel, all function well against Dragons, in all kinds of combinations. Couple them with Brave weapons for better results on the player phase, and mixed-range weapons in preparation for the enemy phase.
Boss Type: Stationary, plus harrassment. Dheginsea uses a wave attack similar to Lekain's, but instead of silencing your units, it damages them. The damage dealt decreases as distance from him increases, and he uses it approximately every three turns once about when one of your units has reached the stairs on any side of him.
Map Tips:
Don't go hard up the middle. The more units you send there, the more Dragons will convene there, and everyone that heads in that direction will be struck by Dheginsea's wave. Better to separate early and reconvene later.
When packing skill combinations like Adept + Cancel, it doesn't matter as much which units you match to which Dragons. Otherwise, physical, high-Res units like Seraph Knights and Nephenee work best for White Dragons, while tanks and bruisers work best for Red Dragons (no magic unit has the HP or Def to consistently chase Red Dragons). Marksmen are excellent on both types.
I like to keep Kurth and Ena near the center until I attempt to defeat Dheginsea. This way, if any situation on either end of the map gets dire, I can make vulnerable units retreat and cover their escape using the Dragons.
Also, Kurth and Ena make great chip units here, since none of the enemies will retaliate. This limits their exp gains, though, so you may want to give one or both of them Paragon.
Taking out Dheginsea:
You need to do >30 damage per turn in order to counter the healing effect of Mantle. It isn't worth striking him unless you can do at least this much, unless you're attacking with Kurth or Ena, since he won't retaliate (giving you free exp).
Marksmen score easy, safe damage so long as there are no other Dragons to threaten them.
Placing Kurth and Ena on opposite sides of a space in front of him can bolster any unit. I still wouldn't recommend anyone without Nihil here, though, because Ire is devastating regardless of whether the unit has the bonuses.
Laguz royals can go toe-to-toe with him much longer than other units. Tibarn, Nailah, and Naesala even occasionally avoid his attacks, but this is rare and you shouldn't bank on it.
Units that shouldn't even try: Trueblades, Marshalls, Seraph Knights, Sanaki, Sothe, anyone with 26 Attack Speed or less. They are generally more of a liability than a help.
And reminder: no effective bonus against him.