First (minor) nitpick...
i run the 20man cheap blob up the field using the crimson slaughter FNP formation rule from the apostle to make it a scary distraction unit
If you mean the Brethren Of The Dark Covenant formation, that means you've got an illegal army - or at least an unbound one.
The formation requires one Dark Apostle and one Chaos Space Marine Squad.
Which you have - but if they're part of the formation, they're part of that 'Brethren Of The Dark Covenant formation detachment'.
Which means they can't simultaneously be part of the Combined Arms detachment - which means that detachment is shy one compulsory troops choice.
......Actually, forget I said that. If it's Crimson Slaughter, I assume they still get Possessed as troops. Never mind. Still, if only just restarting this edition I thought I'd point it out; you've got three detachments in that army list, not two, so I thought I'd flag it up.
anyway..... Yeah. The supremacy suit is rough as hell to face. It's incredibly tough, and (unlike a Riptide or Stormsurge) it's fearless, so even getting a wound on it and making it fall back isn't an option.
You're quite right that trying to kill it with your list won't work. But nor will a big 'distraction carnifex' squad.
I'm going to guess that you set up the Brethren of the Dark Covenant like normal, moved up the board, ran once, then took a Pattern Bombardment salvo from that thing's shoulder guns that has ignore cover to bypass any cover save, S8 to bypass feel no pain and AP3 to bypass power armour, pulverising 60-70% of your biggest, scariest squad in a single shot.
If you're facing something like a titan, big horde units are exactly what they love to engage - they have enough firepower that numbers alone won't cut it, and big enough guns that feel no pain won't do it either. Had that unit of 20 been four units of five spreading out, it would have taken a lot more effort for him to take them all out. Yes, getting first blood would be easier for him, but when you're facing Tau with an assault army, you're never going to get First Blood anyway, never mind when a war engine starts shelling you with supergun rounds....
If you want to compete with a supremacy suit, you have three real options:
1) ignore it. As you found out, this will hurt but it can be done. If you're going to run through tau firepower and have enough dudes left to pose a real threat, I recommend seriously upping the number of independent units and increasing the speed of your army. Bikes, flesh hounds, Crimson Slaughter possessed, rhinos, that sort of thing. On foot, it's turn 3, more likely turn 4 before you meaningfully start hitting back.
2) Close with it and beat it up with your own superheavy. A normal response to a Riptide or Stormsurge (beat it in combat then sweeping advance) doesn't work as the Supremacy armour has Fearless.
The Greater Brass Scorpion is also a bad option here - it's a very good superheavy (and good for annihilating the rest of his army) but it's designed to engage non-superheavies; ignore cover blasts, extra stomps, and flamer template weapons.
For chopping up a Ta'Unar, your best options are the Renegade Knight or Chaos Knight, Kytan, Lord Of Skulls, or the Bloodthirster of Insensate Rage.
The latter is the cheapest, but is susceptible to stomp as it's not a superheavy, and is easiest to kill. At less than 300 points, though, it's a damn good kamikaze unit for engaging enemy titan-esque things. It may draw more fire than it can handle against tau though.
The Lord Of Skulls is as overcosted as it always is, but against a tau army, it at least has the advantage that it can thunderblitz through a skirmish screen of fire warriors or kroot to get at the big guy, and side armour 13 means it's more tolerant of missile pod supporting fire overwatch.
The three variations on knights are pretty unsubtle. All of them can ginsu a supremacy suit in a turn or two, and whilst locked in combat the suit's not firing. Getting to combat is hard, though - you're probably best having two (hence a forsworn detachment of renegade knights is probably best) because losing one on the way in is likely - especially if that thing has the bloody gatling fusion cannons on its arm mounts.
The best, simplest, and fairly fluffy response (warning: note that I didn't say "cheap" anywhere in that sentence!) is a Feral-class Chaos Warhound Titan.
Double-dual turbolasers stand a decent chance of ventilating the thing in one turn. Four turbolaser shots should net four hits*, which have a 50/50 chance of coming up with a deathblow result.
Remember that you, as the attacker, decide the order to resolve hits - with rending, for example, you decide whether the defender makes saves for the AP2 wounds or AP4 wounds first.
Similar thing applies here - if you get a deathblow result amongst your hits, resolve that first. That does (
D6+6)/2 wounds, but more importantly blows out the barrier shield. Against a shadowsword, that's no issue, but against a titan with four turbolasers, that means the other three solid hits get resolved against a target with no invulnerable save, which is likely to be a terminal experience.
* If you're missing a bloody war engine with a BS4 large blast template then your dice need a stern talking to.
