So when you go on multiplayer, there is some button like My clan, Workshop, rankings, and back button. There will be a new button called Quest. 10 steps to complete a quest. After you complete the quest you will get a random mythical.
What to do :

Pay 10 tokens
Step 1: Give the system a big amount of money (100,000+ sm)
Step 2: play against 10 people in a row.
Step 3: earn an amount of sm.
Step 4: Create a clan and get at least 10 people to join.
Step 5: Overheat people 20 times.
Step 6: Drain people energy 20 times.
Step 7: Fight in campaign 10 times.
Step 8: Fuse a weapon level 25-30 to level 5-8
Step 9: Fuse a level 25-30 legs to level 5
Step 10: Fuse a level 25-30 torso to level 10 (any torsos)