Nautilus is a champion hard to kill; that's obvious. With this build, if you play well, you can join and leave team fights easily.

All runes are for life. Nautilus gain bonus shield on Titan's Wrath; The shield are based on the bonus life only, basic life of Nautilus dont give bonus to him (2000 at lv 18).

Focus masteries on gain life and life regen. That make Nautilus so hard to kill in team fights. Of course if you're the focus of the enemy team your life regen will not help you so much, but you can leave the team fight and return without going base to heal yourself.

Eleisa's Miracle will give you 20 mana regen, that will make you keep using your skills while you're in the lane. Will also give you a nice regen life in the begin of the game. I keep it until the late game, but it can be switch for a Sunfire Aegis at late game; You lose some mana regen but will give you life and damage.

On the skill sequence you can focus on Titan's Wrath or Riptide.. Both are good in the beggining. If you want damage you focus the Riptide; If you want shield you focus on Titan's Wrath.

In Summoner Spells I use or Flash/ Ghost or Exaust/ Ignite .. If you're going to support, it's better use Flash/ Ghost; If you want to kill some more use Exaust/ Ignite

Pros:
A huge life regen (About 250/5sec. at lv 18)
A huge Hp (About 6000 at lv 18)
Nice damage with Atma's Impaler(About 85 damage with the 6000 Hp at lv 18)
Good assists in Team Fight

Cons:
Don't kill fast
Need to avoid those who have Madred's Bloodrazor