Because no Black Bolt.
Attempting this with lv191 Kamala, also have a lv168 4/5/4 Quake in the wings
EDIT: took two tries with Kamala, no boosts. First match, Thanos kept cascading into purple and green. Second match, i managed to Embiggened Bash him thrice
Token = my 18th Elektra cover. I also got Elektra on the first day. Seriously?
Newly champed Quake did the job for me, though only about 400 health left at the end.
Gather green and blue, deny purple, destroy purple, Smash with green and get the best matches. He got purple out once, but took it out immediately with a match 4.
3 attempts with Ms Marvel. Used critical boosts to help on her purple.
Token netted me HB black. Champ level.
As with BlackBoltRocks above, it took two tries with Kamala Khan. I got the cover from the event before the second time, just to give her that little extra boost. It helped (along with +2 Green, +2 Purple, and +50% Green Damage) as I ended the match with about 400 health left. I was sitting at 11 green and 7 purple for several turns until I finally got the last match I needed to Embiggened Bash him.
If you haven’t done so already, re-spec KK to 3/5/5, since you have no team members to activate her healing ability.
Token gave me 1000 ISO, as it was Red Spider Gwen #6. Can’t win them all, I guess. 
Failed first two tries with KK 172. Third go passed with Quake 229 2/5/4 b+p/g+be boosted and 50% g+be damage.
Got a green 5* Thanos so now have a 1/1/1 Thanos.
Well, my Quake and Medusa at 4 and 2 covers, respectively, were a no-go. Byt my KK is champed at 181. Got it in one try, no boosts used, but it was very close (KK had about 500 health left). Good thing I remembered to respec her yellow down to 3 since in a 1 on 1 match it’s useless.
Won on the first try with a 170 KK. Respeced as recommended by sambrookjm, and boosted +50% critical damage and +20% match damage. Got a couple crits using her purple, a couple other crits and Bashed him twice. He never got either of his powers off.
Got my first Teen Jean from the token!
died 6 times, before entering 7th time, I requested TUs for Quake and Medusa from alliance, obtain 
& 
and using 
/ 
boost
Bash him, used
, a purple matches for KK’s
, used
to heal KK’s health to 734, select blue tiles to convert to green, and final bash to victory
got 
Done on 3rd attempt with KK.
Got a 
0/3 with Thanos.
Won on 2nd attempt with a champed 3/5/5 KK.
As others have mentioned, respec if you can since her heal doesn’t work on this fight. My first attempt, damage seemed a little slow to keep up with Thanos, so I used some +damage% boosts the second time and that got me over the hump.
I imagine this would be even easier with a well-covered Quake with 10k+ health, but mine’s only 133, so KK worked just fine.
Reward: 
Flaptain #13 (woo!), now 4/4/5, and my 28th max-covered 4*.
was out of the question since I only have one cover for her.
isn’t high level enough at 131 & it’s soft capped at that level, so it resulted me to my championed 
in which I struggled to get enough 
&
matches even though I equipped +2 on them & match damage. Barely scraped it through on my only try. Admittedly most challenging one out of the 3 (Guardians of the Galaxy & X-Men) yet.
Definitely the hardest one so far. Medusa and quake weren’t viable for me as well, so Kamala was the only option, and only one attacking skill (which to make it worse is AOE) made it a pure test of endurance and luck. Managed it 6th try or so only with boosts.
Got a Mr Fantastic yellow for the effort which I could use. I guess it could be worse since I’m increasingly accumulating unmaxed characters with maxed out covers.
0 for 7 with a level 172 Kamala Khan, respecced to 3/5/5, with full boosts. Thanos gets cascade after cascade in all three of his colors every time. **** this battle.
KK at 183, crit boost 100%match, and 2+ all, took 3 tries, got a Fury Blue for a champ level.
3rd try with 193 3/5/5 KK. Added some AP boosts, ended battle with 63 health. Got an Iceman Green, bringing him to 3/12/2 (not a typo).
This. Every game Thanos gets a constant stream of purple matches from off the screen. So much BS.
As expected, this matchup was more of a challenge than the previous two runs, although not as much as I feared. I used my 205 KK: on my first attempted, I boosted the barely useful +125% critical boost, +2 all, +2 p/b, but couldn’t get enough green to Embiggen Thanos more than twice. In my second (and successful) attempt, I equipped an ultimately useless Quake TU, and +2 all and +2 g/b. I never did use the TU, since I got at least 3 Embiggens out to take Thanos down.
For which I received… a black Thanos!
Either I’m bad at Kamala 1v1 or I’m having some bad luck, but either way I’m 6 tries in with my lvl 193 and I’m only getting Thanos down to 5k at best.
If Thanos gets his purple down, I can’t use purple, and he matches me for a bunch of damage. It takes like, 4 greens to kill him, and I’m just not quite getting 48 green before he either gets his purple or green off.
Took two tries. 182 Kamala with max All Damage, G/B damage, and Crit boosts. It was close (666 health remaining). It was the crit boosts that did him in as I got six match 5s.
Reign token gave me 
Third Try KK Level 181 100% GR/BL, BL/PU 50%
Got a Black Thanos 