use the legend affix 2+ all sets from nadroji jewellery or use the elixir mythstone.
The cap is 4+ all sets ( 2 elixir mythstone on amulet and ring or legend affix all sets).
To get ballista (5) though, you place 1 ballista, 1+ (capped 4+ all sets) = (5). This is a way to get the most of a set affix and free up space for many many more set affixes.
To get ballista (6) , place two ballista affix on gear , ballista (2),+ (CAPPED ALL SETS) = ballista (6)