A = Aggressive; NA = Non-Aggresive; L = Links; S = Detects by Sight; H = Detects by Sound; HP = Detects Low HP; M = Detects Magic; Sc = Follows by Scent; T(S) = True-sight; T(H) = True-hearing JA = Detects job abilities; WS = Detects weaponskills; Z(D) = Asleep in Daytime; Z(N) = Asleep at Nighttime; A(R) = Aggressive to Reive participants
Orthrus is almost immune to Light and Dark magic, you should attempt to fight Orthrus on Earth-Iceday to potentially avoid having spells like Dispel/Flash/Kurayami/Threnody as your !!trigger. The real danger in fighting Orthrus is facing Firaga IV followed by Acheron Flames. Stun one or the other if you can, or interrupt Firaga IV with a critical hit weaponskill.
When Orthrus does the Howling (animation), it seems to reset most Enmity.
Orthrus's Howl also grants him an intimidation effect for a short period of time(~15 second)
Critical Hits interrupt Orthrus from casting on a fairly consistent basis.
Due to his casting Blaze Spikes and using Magma Hoplon on a very regular basis, he will have spikes most of the fight. These can be negated by Atma of the Brother Wolf (if you have fought Orthrus before) and Phalanx II. This will also significantly reduce damage from fire spells and Gates of Hades