Is the "Ritual" path supposed to be balanced yet? I was attempting it about 20-30 times before I ended up cheating to beat it (only to realize that there is no real ending for that yet and it does not even award me any ascension points for the previous accomplishments in the story). Not sure if the ritual fight is affected by ascension level, but I just never managed to get to 666 in time. It's much too RNG-dependent with the limited set of cards.
You usually want to get at least 10 shield, but ideally 15-20 to prevent the enemy from getting closer to their goal, since there is no way to revert their progress. If you get a few rounds of bad luck where you don't draw enough shields, you are quickly going to lose that battle. However, getting 3-4 shield cards in your hand won't get you much closer to your own goal, so you need to diversify, making it more likely to get an unlucky hand that leaves you vulnerable.
A way around that problem are cards that let you draw more cards - either directly as "draw + shield" or just drawing multiple cards. However, all of these cost a TF increase on each use, which you need to make up for by adding more penitence to your deck. But those in turn start to water down your chances to draw cards that get you closer to your goal or at least defend you from enemy damage. It's an extremely delicate balance that becomes more and more easily disturbed as you need to add more cards to it. Either you don't get enough shield or not enough damage or not enough penitence to make up for your increase in TF, which locks you out of using cards that would solve the first two problems in the lategame.
Another strategy is of course to rely on the card that adds damage to each card played and then hoping that you get a lot of redraw cards. But in order to get that card at the right time, you need to add a bunch of them to your deck - and that in turn will water it down even more.
Since none of your previous modifications and completed themes carry over into that fight, there is no catch-up mechanic by which you could influence the actual difficulty of that battle.