The party will have some Magic Items. The Paladin might have a nice Longsword, The party will have random Wondrous Items, and probably will all have Cold Resistance at that point.
The Dragon will probably be at the end of a large Ice Castle Dungeon, but they will have a chance for a short rest before the Boss.
Overall the game will likely be moderately difficult, but this Dragon is meant to be Hard/deadly.
The party will know that it is a powerful dragon with breath that will freeze the piss in your bladder (actual line I plan on using) so they will try to obtain Frost resistance. Maybe n Arrow of Dragon Slaying.
The party has no flying, but they have some range. The Beast Master will have it to an extent, but their pet won't. The Warlock will, but not incredibly powerful range, and the Bard might, depending on what spells he choses for Magical Secrets.
SO far Level 9 is seeming like the best time to do it, maybe making them Level 10 as a reward for killing the Dragon.