Finished all the Return achieves. Seasons 4 and 5 really are the easiest to just grind out IMO.
Basically all maps have an "do x amount of events". The Season 4 maps all have bounties which count as events so you just grind those out. There metas also tend to be on short timers.
Season 5 maps are doubled up except Grothmar and Visions of the Past. You just play through Visions once or twice, done. Season 5 episode 5, just play through it. Bjora and Drizzlewood have 2 tabs assigned to them each. The two tabs share progression. The requirements for Drizzlewood are basically just doing the two metas on the map.