There are 3 Aptitude raising methods that don't involve degenerating that I know of.
1.) Farm Goods(Holy Spring, Digimon Lab, and some other one whos name escapes me)
2.) Trading your digimon over Wireless Communication(Requires 2 DSs)
3.) DNA digivolution(You need to do ALOT of this, I had to with my ChronomonHM to have 99 Aptitude)
The Requirements for the Legend Tamer Quest are:
1.) Beaten the Gaia Origin Challenge
2.) Must be a Platinum Tamer
3.) Must have been on Wifi(I'm sure that this is so Nintendo gets more $$$)
For this you will need to beat 9 Tamers in 5 Battles. They have these Digimon:
Battle 1(*Dawn Set*)
Lotosmon Lv.74
Dianamon Lv.83
MirageGaogamon Lv.74
Battle 1(*Dusk Set*)
Angemon
Apollomon
ShineGreymon
Battle 2(*Both Games* 3 tamers at one time)
GeoGreymon Lv.85
Peckmon Lv.85
Gaogamon Lv.85
Battle 3(*Both Games* 2 Tamers at one time)
RosemonBM(Burst Mode) Lv.90
RavemonBM(Burst Mode) Lv.90
Battle 4(*Both Games* 2 Tamers at one time)
ShineGreymonBM(Burst Mode) Lv.90
MirageGaogamonBM(Burst Mode) Lv.90
Battle 5(*Both Games* HINT: Use Any digimon with Burst Counter for this battle, the enemy will do alot of Damage to you, but it may be reversed for even more to itself)
ChronomonHM(Holy Mode) Lv.99
This is the toughest battle in the game. It took several tries for me with Lv.99s. It is rewarding, since you get Chicchimon Data(A requirement for ChronomonHM), which I used to get my own ChronomonHM, and Calumon Data(Only if you talk to it on the Center Bridge after completing this). Plus, you get a Second Digimon Lab, if you got the first one.