Of course Tarquin has weaknesses. He's strong and has pretty much unlimited wealth by level, but he's far from invincible. Even if he did regenerate immediately afterwards, the OotS managed to do a bunch of damage to him in a fight. It's not unreasonable to imagine that he could be killed. He's almost certainly much easier to kill than Xykon.
If I'm reading you correctly, what seems to be bothering you is that, if Tarquin were to lose at this stage in the game, he wouldn't care. That's what the plan he details to Elan seems to say. By being a King for decades, Tarquin already won the only game he was playing at, everything else is icing on the cake. If Redcloak dies without completing his Grand Plan, his entire life will have been wasted. Xykon would likewise be seriously miffed if he bit the dust before realizing his evil scheme. But Tarquin... even if Tarquin were to drop dead right now of a heart attack, he wouldn't consider it a loss. He's playing with house money at this point. If he keeps winning, good for him. If he loses, eh. He already accomplished everything he cared about anyway. It seems to me that it's not any real in-universe invincibility on Tarquin's part that's bothering you, it's the fact that if Tarquin were to lose, he by all indications wouldn't be that upset by it. And I have to ask - Why is that? Who cares how the villain does or doesn't view their ultimate downfall? Does it matter to anyone other than the villain himself? Should it? At the end of the day, dead is dead. The only way that Tarquin is unbeatable is if you accept his view of reality, his definition of "win" and "lose". I can't imagine that Roy, or Redcloak, or even most people, do. In their eyes, if he's dead and his plans are ended, then he's beaten. If you don't like Tarquin's terms, then why agree to play by them?