Nasus

Nasus works backwards from everyone else. Siphoning Strike gains permanent power for every unit killed with it, with no cap: the champion you face at ten minutes has nothing in common with the one at forty. The whole game revolves around that counter, yours and the enemy's.

Wither is one of the most crushing slows in the game, capable of erasing an entire duellist. Fury of the Sands turns him into a mountain for several seconds. In exchange he's slow, has no mobility, and is completely harmless while his counter is low. This is a champion of patience and discipline, not of immediate action.

His scaling has no equivalent. A Nasus who farmed quietly for twenty minutes one-shots a squishy target and takes towers down at an indecent rate. Wither neutralises the enemy's best duellist for the length of a fight, and his ultimate gives him enough health and reach to stay in contact when he should be dying. Spirit Fire cuts armor over an area, which helps his team as well.

The weaknesses are just as blunt. He's slow, he has no way into or out of a fight, and his lane is miserable before the counter builds: any aggressive champion can push him off his minions. He's the favourite gank target because he can't run, and a team that stops him farming simply removes him from the game.

Your lane isn't a duel, it's a job. Every minion killed with Siphoning Strike is a brick laid, every one missed is a brick lost. Don't look for trades: take the poke, position for the last hit, and let the wave come toward your tower so you can farm safely.

Wither isn't an offensive tool at this stage, it's your life insurance. Save it for the jungler who's coming, not to chip a few health points off your opponent. Max Spirit Fire to clear the wave when you're pushed under tower: it lets you keep collecting last hits even from a losing position.

Your first spike isn't an item, it's a counter threshold. From the moment Siphoning Strike kills a caster minion in one blow, your farming speed doubles and the counter runs away on its own.

After that come the items that keep you alive while you swing, rather than the ones that raise your damage, since Siphoning Strike already handles that. Cooldown reduction matters more than raw power, because it multiplies the number of swings you land in a fight. Your ultimate is the last spike: in giant form you can stand in the middle of a scrum that should kill you.

You don't initiate. You walk in behind your front line, pick the most dangerous target in reach, and swing. Your one real decision in a fight is who to Wither: a duellist or a carry slowed that hard is removed from the fight, and that choice is usually worth more than all your damage.

Outside of fights your main threat is the split push. Few champions can hold a tower against you, and the enemy has to send several bodies to stop it. Using that to open the map for your team is often more profitable than looking for the five versus five.