My Doom is ass but I would recommend the following:
Others have already said it, but learn the buktooth loop and his basic corner loop. If you're within a half screen of the corner when you start, the buk loop will push them far enough that you can transition to the corner loop. Once you've got the basic corner loop down, practice the dash cancel version which will allow you to get one extra rep of the loop in, boosting damage a bit, and making the timing on the OTGs less strict.
Good videos:
http://www.youtube.com/watch?v=wDOx4druFsk
http://www.youtube.com/watch?v=3yPskuh1yx8
You should also practice transitioning into the buk loop off of a back throw (I OTG with S.H and the use fwd.h hard kick to launch them). If you want to keep it simple, it does almost the same damage to just OTG S.H immediately into sphere flame. You can combo off of both grounded and air front throws in the corner, so practice that as well.
Doom has good mobility with tri-dashing. You can get in by super jumping, firing the slow moving qcb+L projectiles and then dashing in behind them as cover. When you hit someone with his fwd.H foot dive, you can cancel into a forward air dash, hit them with J.m while still in the air once, and then transition into a normal combo once you hit the ground. If they block and adv. guard, you can time the air dash to negate the adv. guard, but the timing depends on how early they adv. guard. On the ground, you can negate adv. guard by dash canceling blocked attacks into forward dashes to keep pressuring. Now in ultimate, you easily OTG into a combo from hitting a random S footdive, so abuse this to make sure your opponent respects the space beneath doom.
Your standard low block string should be C.L, C.H. Don't use the crouching M, as it can whiff depending on spacing. If the crouching H hits, launch and combo them- if it's blocked, dash forward to negate the advancing guard and stay on their ass.
When doing the buk loop, in vanilla you would normally end with a sphere flame hyper to maximize damage, but hyper finger lasers seems to better for more situations now. For DHC purposes, you should definitely launch once more after the second buk rep and do M, M, F.H, M xx photon array, as it leaves them in block stun for a long time and you can DHC into almost anything as they're falling down. When doing sphere flame -> x-factor -> sphere flame, you should x-factor and do the second sphere flame as rapidly as possible to maximize damage.
For zoning, doom can gain control of a lot of the screen by doing finger lasers from super-jump height. I like to jump and then air dash up, which still allows me to call assists if I want to. In vanilla, you could also do one finger lasers in the air before falling, but now in Ultimate, you can finger lasers, fly, and then do two more finger lasers, enhancing dooms lame game. As you land back on the ground, your opponent should still be locked down from the air finger lasers, and you can safely get out a plasma beam to chip them down and keep them from calling assists.
Doom benefits A LOT from lockdown assists (like how clockwork uses ammy) to force difficult to block high/low mix-ups. Doom also likes assists that help with controlling the opposite top corner of the screen where opponents can safely avoid his projectiles and offensive pressure. Vajra is great for this- strider+doom, bff! J.M can cross up, and you can also use J.M as an instant overhead on some characters. You can also set up cross ups by coming down on the top of your opponents with the new improved ultimate S footdive. Doom can now control the mid-screen pretty well his improved J.H gun, which has the added bonus of being a normal, so that you can still call assists while doing it. The can be a good way to transition from zoning to rushdown.
If someone who actually knows Doom sees any mistakes, please let me know! Good luck!
