To supplement this anon's answer:>use effective attacks to break barriers
They mean elemental bullets. ie, use Sun bullets (Reimu has several) to break barriers on enemies weak to Sun. Effective attacks, called "effective against" in-game, is a way to denote when an attack will automatically crit an enemy. For example, Reimu's first spellcard will always crit against fairies. As an advanced note, effective damage crits are boosted by crit atk buffs, which can really raise your damage against certain enemies.>and when the enemy unit has 0 barriers they lose all defensive buffs
More accurately, they lose all buffs and debuffs. It's a good idea to wait until after you've broken an enemy's barrier to debuff them with skills, since you get the debuff damage on top of the barrier broken damage.>Reimu and Keine are very good tanks. Okuu is an excellent damage dealer followed by Meiling, Youmu, Remilia and maybe Rin
The units anon listed are ones found in the general pool; There's a non-zero chance you've rolled a limited (festival) unit that can be just as good.
And looking at units purely from a damage-dealing perspective is reductive. Many units can deal a ton of damage in the right conditions, it's just a matter of setting them up correctly. For example, Reimu is both a strong defensive unit and also has a powerful LW, as long as you give her story cards that build up her yang atk (Ordinary Magician, Yakumo Family). Raw damage dealing will only get you so far, since most of the difficult content in the game is designed around breaking barriers with elements.
Also I prefer this tier listhttps://東方ロストワード.com/?p=6600