After countless times fighting him, I figured out his patterns and weaknesses are....
1) Sonicbooms - Cancel with your fireballs, or just do a short jump if you don't have one. Be careful of EX version, as he shoots out 2 instead of one Sonicbooms.
2) The long arms - Block (it does not cause damage) or if you're daring enough, do a quick foward HK jump on him.
3) Tyrant Slaughter (the shoulder charge) - That thing is WEAK LIKE HELL. If you block it, he's in trouble as it has a long (and I mean LONG) recovery. If he does the EX version, block it and get ready to pummel him with a Super / Ultra.
4) Wall jump - When he does that, block high - he always try to hit you with a jumping punch.
5) Triple air steps (something like Chun-Li's air stomp) - Just block high. Never block low.
6) The "absorption" thing - This move cannot "catch" you when you're in the air, so jumpkick him when he does this move.
7) Teleport - If he teleports right beside you, JUMP UPWARD and do a HK. He'll always do the Piledriver right after reappearing beside you.
8) Piledriver - NEVER, EVER LET HIM GET BESIDE YOU AT ALL TIMES. If he tries the Teleport > Piledriver technique, do a UPWARD JUMP and HK. At least you have a 50% chance to hit Seth out of his Shoryuken with the HK rather than lose a chunk of your life with this (uncountable?) move.
9) Shoryuken - No real tip on how to counter / defeat this move; the best way to avoid it is by not jumping into his Shoryuken range.
10) The multiple kicks move - Has very short range. Just block. Beware EX version, as he will do the initial kick TWICE.
11) Super / Ultra - This move only hits you ON THE GROUND (as far as I remember, I was never hit by this move when in the air). If you are landing from a jump, and he does the Super / Ultra, just press and hold Back (block) and this move will not be able to harm you.
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Hope the above tips help.