How much DP do you need not to die within 30 seconds of Kutum's lightning attack ?

If you are referring to the attack that applies the DoT debuff its not DP dependant. Its direct hp damage.
You are meant to go to the correct room and cleanse it.

My 338DP Nova alt still gets 1 hit by attacks that dont show any red circles on both SB Karanda and TC Kutum. The attacks arent invisible so its still possible to dodge them if they arent too desynched, which they often are, but still.

These bosses are just so bad that they arent worth doing unless you want the last hit titles or some specific loot they give. Just use alts and focus on your "objective" then skip them once done.