Is a +Atk -Def Eldigan worth it?

He's completely outclassed by his son, who is 4* available. If you don't like him as a character I would suggest a budged build for AA.
Moonbow, Fury, Swordbreaker, Hone Cavalry

Edit: Haven't looked at the IVs, Moonbow should be superior then

Eldigan isn't exactly practical when compared to Ares or Siegbert, but the old dog can still pull his weight quite well.

Weapon: Mystletainn (+Effect)
Assist: Flexible
Special: Aether
A Skill: Fury 3
B Skill: Renewal 3
C Skill: Flexible
Sacred Seal: Heavy Blade 3
Blessing: Water (HP/SPD +3)

This is what's known as the "Fury 6" build, or as I like to call it, "Eldigan's Fucking Pissed". With Fury 6, Eldigan hits a 61/34/41/29 statline, allowing him to cleave through tons of units. However, thanks to this, he takes 12 non-fatal damage after combat. This is where Aether and Renewal come in. Mystletainn's increased special cooldown and 61 Atk Heavy Blade allow him to easily fire off Aether like it's no tomorrow. However, he's still very weak to mages with 29 Res, and he's still rather slow with only 34 Spd.