Hello,
There is no point of increasing the drop rate of just some boxes. Increase them all, or none. This is your personnal odds to get things, for you it's those boxes which are hard to get, for some other players it was other boxes. Currently, all boxes have the same drop rate.
But anyway, just do as 90% of player that got this achievement, afk pet on elite There is obviously no sanction for it since some GS do it / did it / adviced to do it.
Also, this has been discussed in an older topic when Jordan increased the success rate of all crafting achievement, and there was no will to make this achievement easier (Patch v91).
This was Jordan's answer about it :
Source (keep aside the few fight we had while debating) :
Best regards,
Aky