Let's be real for a second: tyranitar is one of the toughest Raid Bosses in all of Pokemon GO. Even if the rumors are true and the Raid Boss of each Tier Level will be rotated every couple of weeks, Tyranitar will continue to be one of the more difficult to defeat. The main reason for this is because Tyranitar is not only a Level 4 Tier Raid Boss, but rather because it eclipses a combat power Level of 34,000 as a Raid Boss.
Because Raid Boss tyranitar has such a monumental combat power Level, it's a must to have a good number of participants apart of the Raid Battle with it, preferably at least 8-10 people helping in the Raid. From there, all players participating in said Raid should try their very best to each load their Party of Six Pokemon with plenty of Fighting Type Pokemon. machamp is the most common counter to Tyranitar, especially ones with the Charged Attack of Close Combat, since Tyranitar is especially weak to Fighting Type Moves. heracross is also a great option, but since it's only found in certain parts of the world, Machamp is used far more often.
Outside of machamp and heracross, dragonite and vaporeon are also very good options with taking down tyranitar. Dragonite usually has enough strength and bulk to do some real damage, but since it's also weak to Rock Type Attacks, players should watch out if the Raid Boss Tyranitar has Stone Edge as its Charged Attack. Vaporeon is far more consistent, as its Water Type Attacks and Defensive Bulk sees it unloading loads of damage to Tyranitar.
No matter how many players are participating, no matter if their Pokemon have a typing advantage, no matter how high-leveled their Pokemon are -- Raid Boss tyranitar will ALWAYS pose a number of challenges to any Pokemon GO Player. But with enough teammates on your side, even this Raid Boss God can fall.