A note on the stacking of elixers. When elixers stack, you are correct. A x4 and a x3 do not make x12 nor x7 but x6.
However, the double XP from double XP weekend (i.e. NOT an elixer) actually fully doubles what you have based on your elixers.
So, a thrasher + a 3x combo on double XP weekend will net you 12X (x6 times 2)
Although this is not technically an elixer issue per se (and your guide is on elixers), might be good to add this little bit anyhoo...

On a side note, this means the best XP gain one can get is stacking a week long elixer with a thrasher with a 3x daily for a whopping 14X on a double XP weekend... (not accounting for slight adjustments based on your level compared to the threat and (is that still there?) the bonus for doing a map with a full group (5 players)....