Rammus' great strength comes with the element of surprise. If Rammus isn't in your face, and he isn't powerballing, then he's not scary.
First, the general Powerball gank-tip: start charging it before you actually go in to gank. It takes some time to get spinning, but lasts for quite awhile. If you just press Q and charge on in, you give your opponent precious escape-time. Figuring out the timing on this is critical.
Second, wards. Namely, destroying their's. Rammus late-game can be something like Fiddlesticks; if you can get the jump on them from a surprising position, you've really got something. Have an oracle's on your team (either you or your support) and clear out those wards. Leave them guessing.
Third, scaling MR blues. Rammus needs MR, and these provide a healthy chunk.
Fourth, effective team communication. When Rammus goes in, he goes in. He isn't getting back out. If your team is not prepared to jump in after you, you're going to have a bad time.
Fifth, make use of your early-game advantage. Rammus is FRIGHTENING during the laning phase. If you manage your Powerball correctly, and your lanemate has any CC at all, you're pretty much guaranteed a kill. Gank frequently.
Sixth, give your lanemates the kills, whenever and wherever possible. Don't ever let a kill get away, but if the opponent is certain to drop, give it to your lanemate. They can put that extra gold to much better use.
Contrary to popular belief, comebacks in Dota2 are much more common than in LoL. Teams with a 10% gold lead 12 minutes into the game (generally about when the laning phase ends):
The main reason for this is that more expensive items in LoL are MORE cost-efficient than cheaper items, while more expensive items in Dota2 (while often having powerful effects) are less cost-efficient. So by buying these cheaper items, you can finish them sooner and have an advantage over your opponents who are building (and have hopefully not yet completed) higher tier items.
As a team who has lost the early game, you need to recognize this and attempt to get back into the game by purchasing mid-tier items of this sort. Some good examples are Bracers, Drums, Urn, and Mekanism. Carry items in this vein are Diffusal Blade and Maelstrom. For example, two Bracers cost 1050 gold and give 12 STR, 6 AGI, 6 INT, and 6 Damage. An Ogre Club costs 1000 gold and only gives 10 STR (with the advantage that it builds into excellent lategame items like BKB).
While you won't have the item-effiency (per slot) if the game goes on for a while, your goal as a team is to make that disadvantage irrelevant by forcing fights. You will have a period of time where you have equal or greater item effectiveness than your opponents who are presumably saving for more expensive items, and that is your chance to bring the game back into your team's control.
At the beginning of every game you should take a look at the hero lineups and figure out who has the superior lategame team. If you have the advantage in lategame and aren't incredibly far behind, it's usually advisable to simply farm and defend towers at all costs to prolong the midgame. However, if you are incredibly far behind or have equal or worse lategame (usually the case), then you should focus on ganking- better equipped heroes will farm faster than you.
Your best option is usually smoke-ganking as a team. You can catch an opponent or two out of position, kill them, and take an easy tower, Roshan, or teamfight with your newfound hero advantage. Just a couple of these occurances can often completely turn the tide of the game as you force opponents to take teamfights that they aren't properly prepared for. After winning these fights, you can place offensive wards, counter your opponents' warding attempts, and continue to lock your opponents into their base and assert map control.
Good luck!
Best Answer
Anything over a Penta kill is called Legendary Kill.