'Play 5/10 games of TFT to Round 20' missions suck.
In the Teamfight Tactics missions, there is are two missions that require you to reach round 20 for an X amount of times. This sucks. The reason? Because to complete the mission is unoriginal, punishes bad luck early game and does not reward people for getting further, or even winning the game. Currently, it would be the most efficient strategy to reach round 20, and then lose after which one can start a new match earlier.
The other missions that require certain classes/origins are more fun as they make people experiment with those classes/origins and build around them. The other missions are mostly luck-based, but unique and not too hard to complete with a solid plan. (Save money and reroll for 3 same champs in a round. etc.)
Now my suggestion would be to change to: 'Complete 200 rounds in TFT.' This would still require players to play 200 rounds, but wouldn't make longer games be as punishing anymore and would make the players that are having a bad day still see some progress being made.
If this is too easy, the rounds could be increased to 250 or changed to something such as: 'Win 100 rounds in TFT.'