Alright, I finally decided to get off my ass and grab some damage averages for Sesshomaru, so here they go!
First off, it all depends on what you're taking. If you take 0 Energy, Sesshomaru gains OHKO damage on Turn 3 with Dragon Strike. No real arguing this; its 740 damage to an average below 200, even *WITH* him factored in.
If you take a 3 turn damage average...he still OHKOs with Dragon Strike, only now its marginal instead of overkill (740 damage to a 230 average with him factored in.)
If you take full Energy, his damage is 2HKO with Dragon Strike. KEEP IN MIND this factors in 3 Energy Attacks that can only be used once with any real consistency, barring maybe Kagome who can turtle to use it again with healing. Either way, its essentially the same as considering 100 FP Attacks in Wild ARMs games as the standard, so I'm inclined to not go by this average.
For the record, energy is similar to Grandia 3 SP, so seeing everyone start with full Energy is reasonable. Just note that a few moves can actually be used more than once before requiring recharging (wouldn't you know it, Dragon STrike is one of those moves that can be used repeatedly; its silly, really.)
Regarding his stats...
Sesshomaru has 250 in every stat but speed, to an average barely over 100...*WITH* him factored in. He has 777 HP to an average of...something not even a 1/3rd of that? His speed is 28 to an average of 22 with him factored in (oooh, look! A stat he's merely awesome in instead of sickeningly curve breaking!)
Chipping Ability? I think staff of two heads does less than his physical, but its still probably above average damage anyway knowing him -_-; However, Poison Claws I think adds Poison status which should be enough. He can also critical hit with anyone of his skills, and its likely this will happen long before Terra can take him down. Do not underestimate Sesshy's durability; having nearly 150 points above average defense at worst, in a subtraction system, combined with about Boss Like HP on a PC...
Well, best way to put it? He's IRT Kaiser...without an upkeep, when it comes to stats.