AC should be partially realism based... giving monster appropriate equippment to achieve this... better... you should be able to guess from sight, which one is the tougher opponent maybe.
I think, that 4e did a little bit too much to balance monsters. You can use the roles... but if you look at newer books, not all artillery share the low AC anymore, but some lurkers do.
Monster roles may also be changed a bit in 5e. I hope, 5e has assigning monster roles just the other way around (actually I do design them that way):
IMHO you should assign:
AC, HP, Defenses, Attack bonus, Damage expression ranged, damage expression melee, consult a chart with appropriate level for the monster. Dress it up with PC advancement rules, and you get a certain number of hp, armor etc., look at the chart and know how much xp it is worth.
Guidelines should help:
Just say: here are 5 possible combinations that result in balanced monsters that can fullfill a cetain combat role very well: brute, lurker, skirmisher...
i hope you get the point.