If you are looking at this mob I'm guessing that you are trying to solo a codex, like I was for five hours this evening:
These are MNK mobs, so they hit quite fast and conter ~10% of the time.
The drop rate for codex is supposedly ~5%, I have been here five hours WITH TH and killed about 40 of these, with no drop.
I went NIN/THF, and loaded up on evasion. They will miss in this case 60% of the time.
They take about 2~3 minutes to kill, quicker if you use elemental ninjutsu, which they are very weak to.
I used Blade: Retsu for a strong Paralyze effect that procs more than 50% of the time on these.
Mortal Ray is easy to avoid IF YOU CONSTANTLY PAY ATTENTION TO YOUR LOG WINDOW. Make sure that you turn off autotarget so that if it redies MR you can turn away immediately.
His other tp moves are Mow, which attempts a 12pt/tick poison, and a triple attack. Also Unblessed armour, which is Shell.
Note that they hit me for between 40~60 points of damage.
Hope this helps.