I've been playing Tremor relentlessly and right now I don't feel like there a match up he can't actually fight. I use Metallic because I feel it deals with the most match ups, but from my experience.
The obvious one is he is lacking in speed, but with D1, D4, and D3 it does not leave him helpless. D4 Does great in any match because it's near impossible to be able to tell exactly how big the hitbox is when Tremor slams the ground and with it being special cancel-able you can do three things. One which is the easiest is Boulder drop, but does not work against characters like Reptile. Two is Rock punch which works against everyone, but it's not entirely safe so careful how you use it. Third and the best/hardest option is special cancel the special move with Rock punch or Rock shatter with the cancel command respectively. Dunno if it leave him plus or not, but I'm sure it will leave your opponent shaken at the very least.
Next is his pressure game. You can use 21 to if you land a hit with D1, D4, or D3 first. If several steps away you can use F1 which can be ducked easily, but his fast move or B1 which has a long start up, but stuffs low profiles. B32 is not a bad spacing tool, but still carries the HIGH RISK for the same fact opponents can jump over it. I also recommend not completing the B22 cuz his overhead is EASY to react to, but the third low is faster so the likelihood of B322 hitting is not good just special cancel at B32 and stay safe. 212 or 21D4 are his main mix up tools and thankfully not easy to punish. His 21D4 however special cancel-able so you actually use the same tech as mention above this paragraph. I've experience matches where I would get 2-3 string out cuz of the Rock punch or Rock shatter cancels.
DO NOT use 21 as a punch or spacing tool. 21 is Good for many other things like anti-air, pressure, mix-ups and most importantly combos. B2 is a really good spacing and anti-air tool, plus the inital frames has Tremor ducking so he can go under certain attacks. B2 is of course special cancel-able so you option are just really good.
Boulder Drop is best used at full screen, set-up after a combo(I prefer my final hits to be 21), and any special cancel-able block strings. Reason full screen is best because if you opponent approaches they are COMMITING to using advancing attacks so it's easy to tell where to drop the rock, plus you can hit buttons. I recommend D4 for the fastest runners, but for others use B2, B32 or B12. Block strings last cuz although I would say use it in the early part of the match. Use an armor move if necessary. I would not rely on this unless your CERTAIN you opponents gonna fall for this trap. Again best for opponents who use character that don't have attacks that moves them around too much.
As for Tremor Variations(I shorten it down to "Var.") I prefer Metallic overall, but I would not snuff out the other two. Aftershock I havent messed with yet, but Crystalline is probably the best at dealing with CQC. The reason is really just the crystal shatter. All tips above applies to all Var.s, but close up Crystal DB1 is the best cuz he is only -4 at best and does the same chip damage as Metallic Lava form Shatter lava, but Shatter lava is unsafe on block.
Crystalline does suffer of not having a immediate option to deal with zoning. Good for fighting Cassie who is forever plus I swear or Scorpion who can be punished fairly easily for example Scorpion does B32F2 then backdash Tremor punish with Crystal shatter(DB1D) .
That where Metallic comes in as a great COUNTER zoner. Metallic GOLD can armor up his Gold Rock toss and shoot it and all need directions, but when the opponent blocks or is hit you run up to mid-screen and threat another toss or something else.
Again I have not checked out Aftershock all too much. Too me it seems to be the lesser of the three, but I dunno. I would just pick Crystalline for rushdown opponents and Metallic for more flexible opponents.
There are many things you can do, but for the most part you need to block when your pressured and when you are free to attack you have many methods to attacks. Don't limit yourself.
