The reason I said "especially for a pokemon with such bad defensive typing" is because it has a total of 2 resistances, 5 weaknesses and 0 immunities wheras Ferrothorn has 9 resistances, 2 weaknesses 1 immunity to an attacking type and immunity to being poisoned and badly poisoned. This has a profound effect on Kyurems bulk, and is the reason why Ferrothorn is so good at walling things.
252 SpA Alakazam Focus Blast vs. 0 HP / 0 SpD Ferrothorn: 238-280 (82.3 - 96.8%) -- guaranteed 2HKO
252 Atk Terrakion Close Combat vs. 0 HP / 0 Def Kyurem: 426-504 (108.9 - 128.9%) -- guaranteed OHKO
252 SpA Alakazam Focus Blast vs. 0 HP / 0 SpD Kyurem: 294-348 (75.1 - 89%) -- guaranteed 2HKO