Since we're not able to burn down 5th boss yet we do the strat which lies the problem of Thallsus targeting you with one of his rays more than once every 2 mins. If you're a warden or a class than can absorb flame ray you're fine but not every class can, therefore using cake is necessary (freeze requires cake no matter what afaik). Cake has a 2 min cd but that doesn't mean the boss won't target you twice in the cd phase. As far as we know, there isn't a way to get out of freezing ray if you don't have cake without dying and having a lock combat rez you, if there is a way please tell.
Basically a suggestion for this boss is to make him not be able to target the same player twice in a two minute timer or go through the whole party first, that way he rotates around the party which allows for different people to position themselves correctly when targeted, rather than praying that Rng won't pick them twice. At the moment this boss is based on luck and has a chance to just screw over the party because of freezing ray and Rng, which is imo a horrible boss mechanic. The argument of "using nix" is not viable because you can still get frozen meaning if you're healer, party = screwed.
tl;dr Change Thallsus ray to not be able to target the same person twice in 2 mins or at least rotate through party first so you can do the strat properly rather than pray to Rng-eesus.