By the Hextech gunblade, 6% Spellvamp by Runes and masteries, 25% Healing empowering through Spirit visage and 8% bonus healing through runic armor you get a lot of heal, also you have high defense values, thus you get healed by auto attacks while your w is active, also the Thornmail heals you through all the effects named beforehand (except for the 6% spellvamp).
Also with the Hextech gunblade and the Blue Jungle item, you can slow your enemys to get in range to taunt them. You really wont get any damage by autoattacks and your ult heals high amounts in teamfights, it is possible to fight 5v1 for almost 15 seconds, also you deal lot of damage. You don't have to use your brain actually; just go in, select a target with your e, press w and kill it.

With your items you need 20% cooldown reduction, as your runes will give you 20%cdr, and the more cd u have, the better. So you get spirit visage for the 10% and it gives you additional heal, also you get abyssal scepter for the other 10% cdr and the extra dmg on your w (and r) also you dont have any mr in the runes, so you need these 2 items for mr aswell. Thornmail should be ovious. Dead mans plate provides you with walk speed, so you can succesfully chase, also it gives health and armor. The Gunblade also helps you with the chase, as it's active slows enemys. (also you built the blue jungle item to slow the enemys so you can reach them, but replace it late game with the dead mans plate, when you have a high lvl and have all items but dmp). The Ninja Tabi are also a good choice because when you taunt the enemys you will get hit by auto attacks.

The more enemy minions surrounding you while a fight, the better:
You will get healed by 20-60 HP/per minion hit when you are full built as described and with your w active.

Focus the mage:
It's quite obvious, a caster wont get dammaged when he casts on you, so roll into the strongest caster, put your e on him, press w and r and get him killed fast.

Get healed by Auto attack based champs (abc's):
As long as an abc doesn't have the percentual armor penetration item, they will heal you with their auto attacks with your w active, so in a fight, never focus them.

Pros:

• All Champions which auto attack based (ADC's, Yasuo, Xin Zhao, Master Yi etc.) actually deal less damage to rammus, than he heals by their auto attacks, so if 5 of them attack you in a 5 v 1, you will get a pentakill by activating your w (but it almost never accures though, that the whole enemy team has abc's)
• Often deals the most damage of the team, without being squishy
• It does not take a lot of games to learn
• Counters all champions who don't have strong escapes

Cons:

• Not much of a challenge to play