Is it true that high dosage acetaminophen or paracetanol can kill you when mixed with alcohol?

In high doses, yes. Too much alcohol is damaging to the liver. Too much Acetaminophen is damaging to the liver. The two together have a synergistic effect, which means the two together are more damaging than high doses of either one alone. So very high doses at one time could be fatal, and moderate doses over time with the two may cause eventual liver failure.
Yes. They can kill you without the alcohol. A toxic dose of Acetaminophen is about 150 mg/kg, which is 10 grams in an average sized man. Liver disease may make the amount needed even less, and alcohol use transforms the drug into a more toxic form, so less is also needed. As little as 4 grams in alcoholics (8 extra strength tylenol) can result in death or serious liver damage.