Bennett, Trial by Fire (命运试金石), provides one of the best ATK buffs in the game with his burst.
This is one of the best bursts in the game.
His burst heals and provides a broken ATK buff. The ATK buff scales with Bennett's base attack (weapon ATK + character leveling ATK). The burst costs 60 energy and lasts 12 seconds. The radius of his circle is 6m (aoe).
The burst will also apply pyro to those inside, so it can remove things like Azhdaha marks and some ley line effects. When paired with Jean's burst (which applies Anemo), it can create the so-called 'sunfire' effect, which is great for shredding lector shields.
With one of Bennett's passives, the cd of his E is reduced inside his Q. This makes it easier for Bennett to spam his E more in his Q. Both his E and Q apply 2B pyro on cast, so Bennett is simply one of the best characters for shredding Electro Lector shields.
Several abilities can snapshot Bennett's ATK buff when cast inside the circle. But not everything does (like Xingqiu's Q or Yae's E).
For a moderately invested build (2000 ATK on the dps character) and a maxed Bennett (c5, lv13 q, aquila sword, lv90), his buff can increase damage by around 1.5 times (formula). If there is any 4 star burst worth crowning, this is it (unless Genshin releases a Bennett 2.0).
Bennett's key constellations are his C1 (which removes the HP requirement for his ATK buff) and his C5 (which adds 3 levels to his Q). Many players would just stop at his C5 and not activate his C6 due to its anti-synergy with some characters.
Bennett works with almost every character and team in the game. He does have anti-synergy with Hu Tao as healing her will make her lose buffs. And Bennett's C6 which gives a pyro infusion can mess up some characters such as physical carries like Eula and Qing as her electro infusion will be overridden. But for most other characters whose infusion cannot be overridden by Bennett's C6, his C6 is not the end of the world. It even gives a modest pyro dmg bonus, which is good for pyro carries like Diluc or Yoimiya.
Note his C4 will allow his charge level 1 skill (usually 2 parts) to do an extra 3rd attack if doing a normal attack during the 2nd part of the E animation. This makes it similar to his charge level 2 (which has 3 parts) with no launchback and shorter cd. However, Bennett's charge level 1 or level 2 skill is almost never used. His skill is almost always pressed/tapped.
4 piece noblesse. Can go damage route with ATK-Pyro-CRIT or support build with ER sands and HP goblet or circlet. For substats, ER is good. ATK, CRIT, HP are fine too.
There is a meme trick of using 4p Thundering Fury with Bennett. The idea is when Bennett procs overload (on say a lector shield), it will reduce the cd of his E by 1s. Bennett already gets a lot of cd reduction from his two passives. So essentially 4p TF Bennett can spam his E once every 1s. So he becomes like C6 Xiao almost.
Bennett wants swords with high base ATK. Having ER on the sword is nice too. Aquila Favonia (for high base ATK) and the Skyward sword (with ER) are good 5 star options. For 4 stars, high base attack ER swords like Festering Desire are fine.
How much ER is enough? For a comfortable build, 200% ER is enough as a benchmark. Note Bennett ascends with ER, so 200% ER is attainable (especially with an ER sword). Depending on the situation and team, he may be able to get away with less or may want more.
Calculations are for a melt Bennett. Bennett is best used as a support. He can do decent damage, and his q can buff himself, but his q also buffs other characters.
At lv13 his Q has 494.7% scaling damage. Suppose Bennett has 61.6% pyro dmg bonus and is on 4p Noblesse (for 20% more burst dmg). The cd on the Q is 15s. Assuming he can spam it on cd (and melt it) the burst dps is $$x_Q = 2*494.7*1.816/15 = 119.78336$$
One passive reduces the cd of E by 20%. Another reduces the cd by 50% inside his Q. The initial cd of his E is 5s. From testing, it appears these two cd reductions are multiplicative. That is, with these two passives (and casting E inside Q), the cd of his E becomes \(.5*.8*5 = 2\) seconds (rather than say \(.3*5 = 1.5\)s which is what you would get by adding 20% and 50% for 70% cd reduction). So assuming Bennett is not on 4p TF (which can reduce cd by a further 1s on overloaded reactions to a cd of 1s), he can cast his E once every 2s within his Q.
The Q lasts 12s, so say he can cast his E six times within its duration. The scaling of his E tap is 292.4% at lv13. The dps from his E inside his Q is $$x_E = 6*292.4*1.616/12 = 236.2592$$ assuming no melts or vaporizes. Note standard icd is 2.5s/3 hits, and since Bennett applies strong 2B pyro on each E tap, he will rapidly deplete any existing hydro/cryo status on the enemy. So in practice, it may be hard or not even possible to melt or vaporize each E without very rapid hydro/cryo application (or if the enemy is something like a hydro/cryo slime or is wet in perpetuity).
The total dps is then $$x_E + x_Q = 356.04256$$ Note this is for using Bennett on field most of the time. This dps is around Rosaria level. But in general, the team will do a lot more damage when Bennett is used as a support, and his Q is used to buff other hypercarries who can do much more dps.
Bennett's tap E can generate 2 particles (sometimes 3 but for simplicity we will assume 2 as a baseline for this calculation). So with 200% ER he generates around \(2*3*2 = 12\) energy for himself per tap E (if on field). So he will need to tap his E around 5 times to fill his energy requirement of 60. As shown above, he can spam his E around 6 times within his Q during its duration, so Bennett can comfortably spam his burst on cd with 200% ER.
Some have remarked that the advantage of dps Bennett is that it frees up a spot for another support. That is, most atk scaling dps will need to spend one slot on Bennett, leaving 2 flex spots. But if Bennett himself is the dps, he gets 3 flex spots.
Another example dps Bennett team is dragonstrike Bennett, enabled by XY. Here, one plunges with Bennett and weaves in Es between plunges. So he is sort of like C6 Xiao or Cyno. Also this benefits from C6 Bennett infusion and pyro dmg bonus.
An example team would be: Bennett-Furina-XY-XQ. And the team can actually be decent.
One can build Bennett on dps/crit build without sacrificing his buff since his atk buff only depends on his base atk (weapon+character level). This is different from XY as doing dps build on XY (crit build) usually means sacrificing a portion of her plunge buffing abilities (ATK-Anemo-Crit build usually means less ATK than full ATK build, meaning less buff from passive).