No Mortal Can Resist Can be used when your Life points are at least 1000 less than the opponent's. All monsters in the opponent's Graveyard changes to Skull Servants (Zombie / DARK / Level 1 / ATK: 300 / DEF: 200). This skill can only be used once per turn..
Since the 2 most popular decks right now are REBD and Nephthys decks, which are very reliant on their graveyard, No Mortal Can Resist is very deadly to them in that it turns their key monsters into useless Skull Servants.
Summon Sergeant Electro and use his effect to target your opponent's set spell/trap cards. Doing this would either lock the spell/trap card from being used while Sergeant Electro is face up or force your opponent to immediately activate it and make less optimal plays. Doing this could be devastating for your opponent if he is using a control deck which usually needs to manage it's resources, or if your opponent is running low on cards.
Use Mirror Wall to halve your opponent's attack points when he attacks your monster. Doing this would most likely destroy his monster and force him to end the battle phase.
When your opponent has 2 or more face-up attack position monsters when he declares an attack, use Magical Arm Shield to take the stronger monster and force the weaker to attack it. This could do a considerable amount of damage when comboed with Mirror Wall.
Even if you can't destroy your opponent's monsters by battle, you can still remove them from the field with card effects.
If you manage to summon re-summon Heavy Knight of the Flame and get his effect active. You can use him to attack special summoned monster and instantly remove them from play with his effect. Very useful for special summon reliant decks like Nephthys, Toons or REBD decks.
Since many decks use some sort of level 4 monster, 4-Starred Ladybug of Doom can help you get rid of them quickly and en masse if used correctly.
Super Rush Headlong can be used for offense or for defense, but it would always be used to destroy an opponent's monster.