Inside the giant mushroom, I have no proplem with the Felucians but suddenly someting explodes and I'm dead. Full health to dead like BAM! Happened a few times with Kazdan Paratus's force pushes to just one hit killed me.
Supposed to be like this?

Well, near the entrance and near the exit (at least) of the giant Felician mushroom there are some plants (not the ones you can grab bomb like fruits from) with some greenish plates on. Some stand by the side or hang from the roof. If you walk/jump into them they explode and do heavy damage. At Sith Master difficulty it is possible they kill you outright. I guess you've encountered those. They are easy to miss since they look just like part any other plant and not even the guide book mentions them. As far as I know they exist only there thankfully.

As for Kazdan if he stands behind some debris or something when pushes at you and hits you clean with both push and debris you'll get multiple hits at once and that sure can "one hit" kill you on Master difficulty.
Personally I find Kazdan one of the hardest bosses at Sith Master.
You like a challenge you can also try the Jedi Temple dlc at at Sith Master if you haven't already. The opening landing scene there is almost just ridiculous. You can be dead before you even hit the platform if you're unlucky.

I had a harder time on Felucia at Sith Lord than Sith Master. I had the same problem you said you're having, but on Sith Lord instead. On Sith Master I found it really easy. Each one of the Felucians can be killed by slamming them against something. For the red guys, just jump in and Force Repulse them. That'll give you enough time to knock everyone else around you away, then chase the red one and stab him on the floor.

Guilty though, I skipped the 3 rancors at the end. Would've done my head in if I had've tried to kill them.

I had the same problem as you on Kazdan, though. It only happened to me once, but I think he might be taking your force power and countering you with it, which is why it's so powerful.

I'm stuck on the landing platform at the end of the level where you go get Rahm. It's unfair to expect you to last so long without dying on such a hard difficulty level. I've tried it three times so far. Got absolutely creamed the first time. Got as far as the Shadow Guard second time but had hardly any health left. Then the third one was classic Sith Master happenings, I got as far as the shadow guard and got shot by a sniper during the *bleep*ing cut scene, so I just dropped dead when the cut scene ended. It's shit like that that irritates me about Sith Master.

If you have problem with the Shadow Guard fight at the platform I have good strategy.

As soon as you enter the last platform run to the other side and hide as best you can behind some pillars and try to kill those annoying locals on their machines by throwing stuff at them. If any jump-troopers fly in grab them and through them preferably straight down into thin air (instant long way down death)
When the locals on the machine is dead the shadow guard will appear, now run back over the platform up back the way you once came. There might be some troops here if you did not kill them all before but up here you can find cover so its not so bad. The troops that accompanied the Shadow guard tends to follow you up there but the shadow guard tend to linger at the platform so by the time you face him he should be fairly alone and the fight way easier. He is fairly sensitive to sith slashes and grapple moves, when he's on his ass you can unleash lightning or run up to him try to stab him.

Ahh, Cloud City level... the 2 things that annoyed me the MOST were those Ugnaughts piloting the gravity-lift thing and Snipers. Notice that the Shadow Guard appears only when you close in to the last platform or kill both of the Ugnaught pilloted machines. The best way is, at first, NOT TO ENTER the area and finish off all of the snipers and Jetpack Troopers that appear in the upper level. Only then should you go down to fight the Ugnaughts. The safest (yet slow) way is to continuously shock them with Force Lightning and when your force is depleted, attack them with a Leaping Slam. That way they won't attack you with the laser and you'll avoid the shockwave they make while they stomp. If a Jetpack Trooper appears while you're fighting the Ugnaughts, just shock him with lightning. Chances are his jetpack will malfunction and he'll make an instant exit from the stage

And finally - The Shadow Guard. When he appears, go get the Sith Holocron giving you Unlimited Energy. Eliminate the stormtroopers and Sniper first and then take care of the Guard. Just hit him repeatedly with Sith Slash until he falls down and then just stab him while he's on the ground. Took me about 2 - 3 mins. to beat him on Sith Master Difficulty.