there are no hard dungeons. however, the extreme primals are difficult, but clearable with randoms... sometimes... the hardest content is the savage mode of the alexander raid. the last turns (9-12) are less difficult than 1-8, but still very hard. you can maybe clear turn 9 with randoms, but turn 11 and 12.... i don't think so.

but however, it is true that you can clear all the content with randoms. but of course that depends on the randoms you get. if you have already cleared a raid-turn or an ex primal you can use the raid finder wich will only search for people who have cleared said content as well. so it is easier to find randoms to clear harder content. but first you have to earn the kill yourself ^^

the raid is really hard. turns 1-8 were cleared by around 1-3% of the playerbase and has destroyed many raidgroups. it will get easier in the future i guess, like 9-12, but it is still challenging.