This makes it a great early play, usually on turn 1, but not really worth keeping around once turns 3 or 4 come to pass
With Mavinda in play, your instants and sorceries that target have a free flashback cost, and that can be used to blink, to cast powerful combat tricks, or to cantrip with a card like Defiant Strike
Certainly it is not useless but it is narrow, situational, dependent, unreliable, capped in potential and generally just pretty weak
Graded Population Report: Sophocles [Reverse Holo] #65 Visualize the distrbution of grades in the chart
This does make Tyvar Kell a bit too slow for constructed formats
You can carry this out as long as the Character you wish to KO has a cost equal to or less than the total of your and your opponents Life cards