Tepeu's max level is 31. You gain his levels by obtaining Life Shards. These shards are blue. Do this by finding the Life Shard chests a well as defeating enemies. You will raise this faster than the Friendship level, but both should be within a few levels of eachother in regards to how many they are away from max. To see what level you are, just hit .
Friendship max level is 21. This is going to take longer than Tepeu's max level since there are no friendship shard chests in the game. To get these shards, you must use combination attacks as well as finsh attacks. Also, during the night time you will find floating "ghost" like enemies in groups. Attacking them will give you instant friendship shards and a pretty fair amount of them.
You will not have this while playing naturally in the game. You will have to grind out the last levels before the final boss. The best place to do this is in Arkela Castle - Hall. This is just a few areas before the last boss of the game. Play the game until you get to the end of the Grand Staircase. Save but do not go any farther. Now, backtrack to the Hall area and you will see 7 enemies. Lead Majin into the little hallway with the save point, and tell him to wait. Now gather the enemies and bring them to that hall. They will for the most part stay at the end and can't move forward unless they were lunging at you. Have Majin use Lightning on them and kill them all within a few seconds. Now go to the save point, choose "change outfit", and then back out. The enemies will have respawned and you can now do this until you have the achievement. This wll likely take you a couple hours. On average you should gain roughly 1200 exp per respawn.
Note: Remember to equip the Bandit's Wristguard and the Bandit's Costume. These will increase the Life and Friendship Shard amounts.