Legends of Runeterra has many card effects that are common enough that they get their own keyword - a word or two that immediately tells you how the card works. Let's take a look at the Overwhelm keyword.
Excess damage I deal to my blocker is dealt to the enemy Nexus.
For a unit with Overwhelm, damage is the name of the game. If their Power exceeds the enemy unit's Health, the difference is dealt to the enemy Nexus as damage. Plowing through a unit with lower Health will ensure that Nexus damage will make it through. (Trample? Never heard of it.)
It should be noted that Barrier does not prevent a unit with Overwhelm from damaging the Nexus.
Keywords that Work Great with Overwhelm
Challenger is a great keyword to pair with Overwhelm. A unit with Challenger is going to be allowed to pick which enemy unit it faces, meaning you can guarantee a low Health unit will stand in your way. This maximizes the damage you inflict to the enemy Nexus.
Playing Against Overwhelm in Legends of Runeterra
Overwhelm requires that their Power exceed your unit's Health, so high-Health units are your first priority. Barring that, Stuns or Recalls can slow the enemy down, buying you time to field a proper defense.
Example Legends of Runeterra Overwhelm Cards
Darius has a whopping 10 Attack after he reaches Level 2, and he can Level Up mid-fight! Use this to your advantage, and position attackers such that Darius goes after everyone else (put him on the far right). The other attackers will knock the enemy Nexus down to 10 or below, ensuring Darius can get the killing blow.
Might gives +3 Power and Overwhelm to any unit you'd like, allowing for a sudden uptick in damage. Use this to trade into a higher-Health enemy unit, or to surprise your opponent with Nexus damage.
Purrsuit of Perfection summons the Catastrophe unit, an absolutely huge 30/30! The enemy Nexus only has 20 Health, so you're practically guaranteed to Overwhelm for a lethal hit.
Savage Reckoner showcases the power of combining Challenger with Overwhelm, allowing you to pick which unit will block. Choose wisely and inflict maximum Nexus damage!