The pack will disappear before then. Once you have all non-legendary characters Phenomenal Packs disappear. Sadly, there are only two ways to get legendary characters.
First, purchase from packs, these seem to drop once per million coins spent from the 35k packs. (These are confirmed by Popcap to be the highest chance of finding one.) The disadvantage is the legendary you get can be a character or a hat. In addition, since the pack merger it can be from any DLC of the game as well as the base packs.
The second way is to wait for RUX to bring the one you want, this will cost 1 million coins per piece, a total of 2 million per legendary. Like before, it can be any legendary you don't have so it will require luck to get the one you want. (It is also quite expensive.)
The exception to this rule is Party Characters which can be found in Infinity Packs (while they are available) or obtained for finishing a round of Infinity Time. (The best results are for 25k shards, though they can drop any time you earn a chest.)
Of course, Z7 Imp and Unicorn Chomper require special conditions to unlock. However, you will have a chance to play and master them sometime this month in the hero showcase. (I highly recommend you and everyone else take advantage of this.)
Good luck.
