What makes your Mewtwo stronger than any other Psychic-type, like Abra or Drowzee? The answer is that Mewtwo has higher stats than Drowzee. But what are 'stats'?
Simply put, stats are numbers hidden behind your Pokemon's model. When you attack something with your Mewtwo, its numbers are put up against the opposing Pokemon's numbers, and the resulting calculations determine how much damage you deal and receive.
Mewtwo, who has very high base stats, will almost always deal more damage than a Drowzee. But what are base stats?
Base stats are fixed, unchangeable values that represent the battle potential of any given Pokemon species; the moves that Niantic assigns to that Pokemon species determines how much of that potential is realized.
In Pokemon Go, every species of Pokemon has three base stats: Attack, Defense, and HP.
While base stats can and do vary between species, they do not vary within a Pokemon species.
Snorlax has base stats of 190 Attack, 169 Defense, and 330 HP. Every Snorlax, no matter its IVs or Level, no matter if it's Lucky or Shiny, will have these same three base stats.
Some species of Pokemon have major aesthetic variations between individuals, known as forms. Sometimes, base stats are different between different forms of the same species - this is the only exception to the rule above.
Mewtwo has base stats of 300 Attack, 182 Defense, and 214 HP.
Armored Mewtwo has base stats of 182 Attack, 278 Defense, and 214 HP.
"So how do I tell what my Pokemon's base stats are in-game?" Unfortunately, you can't; they're not explicitly shown anywhere in-game. A full Pokedex, showing base stats, movepools, and more is currently under construction for this site.
For Raiding, Attack is the most important stat. The 300-second timer is your enemy, not the Raid Boss itself. If you use something that has low Attack and high Defense, like Blissey, you will survive a very long time, but you will also do close to no damage, and you will run out of time unless the rest of your group is strong enough to carry you.
Having high base stats doesn't mean anything if your Pokemon's level is too low.
A level 20 Mewtwo only has the potential to be strong. If it stays at level 20, that Mewtwo will not be very strong in Raids, even if it's 100% IVs. But what's a 'level'?
A Pokemon can be anywhere from Level 1-40. Every time you power up a Pokemon, it gains half a level.
While levels aren't explicitly shown in-game, you can estimate your Pokemon's level via the Stardust cost on the 'Power Up' button. The important milestone levels are highlighted:
When powering up above Level 30, don't just do so blindly. If you can't afford to go all the way to Level 40, it's more efficient to power up to the highest breakpoint that you can reach (What are Breakpoints?).
Repeat after me:
A 100% IV Level 20 Mewtwo is significantly weaker than a 0% IV Level 40 Mewtwo:
This is a 0% Mewtwo. Mewtwo has 300 Base Attack.
300 Base Attack + 0 Attack IV = Effective 300 Base Attack
This is a 100% Mewtwo. A 100% Mewtwo also has 300 Base Attack.
300 Base Attack + 15 Attack IV = Effective 315 Base Attack
Seems like the 100% Mewtwo is better, right? 315 is higher than 300, after all.
Here's the catch: Base Stats and IVs alone don't tell you the whole story. Levels are the final piece of the puzzle for determining what your Pokemon's actual stats are.
'Actual stats' are the actual numbers that are used in real-time calculations, whenever your Pokemon performs an attack or takes a hit, that determines how much damage your Pokemon deals and receives.
Now let's go back to our Mewtwo and account for Levels:
Here's a 0% Level 23 Mewtwo:
(300 Base Attack + 0 Attack IV) x 0.64065295+ = 192 Actual Attack
And here's your 100% Level 20 Mewtwo:
(300 Base Attack + 15 Attack IV) x 0.59740001+ = 188 Actual Attack
192 is higher than 188. What this means is that your 100% IV Level 20 Mewtwo will do less damage than a 0% IV Mewtwo that's just 3 Levels higher.
If you want to learn more about the actual impact of IVs, see our more in-depth Explanation of IVs.
+This is the CP Multiplier, a number that is used to scale your Pokemon's stats with its level. Check out Advanced Pokemon Stats for a more in-depth explanation on the CP Multiplier.