In some behemoth fights, it is very helpful if you can interrupt or "boop" a behemoth. That will knock down the behemoth and leave it sprawled out on the ground for several seconds, giving the entire party a chance to safely hack away at it. Some behemoths have an unstable attack (or occasionally two) that can be interrupted by damaging the behemoth with a "heavy" attack of your own during its unstable attack.
The problem with this is that some attacks count as "heavy" attacks and some simply don't. If you hit a behemoth with an attack that is not considered heavy, that won't interrupt it. When you're trying to interrupt the behemoth just before it hits you (e.g., Embermane), failing to interrupt it means that it hits you. So you really need to know which of your attacks can interrupt it and which cannot.
The problem is that there isn't that much good information out there about which attacks will interrupt and which will not. Much of what is out there is contradictory, and it appears that various patches have changed whether particular attacks count as heavy. Some information out there is thus obsolete, which explains why it is so often contradictory.
My solution to this was to say, this is an empirical question, so let's go try it. Skarn is by far the easiest behemoth to interrupt, as when it stands up, you have a few seconds. It stands still, leaving no doubt as to whether you hit it or not. An interrupt will knock it over backwards. If it falls forward, you didn't interrupt it. Whether or not you actually landed an interrupt is pretty obvious for all behemoths, but some move so fast that it's not always clear whether you landed the attack that you were actually trying to land in order to interrupt a behemoth. These were solo hunts, so when a behemoth was interrupted, there was no ambiguity as to whether I interrupted it or someone else did.
I generally tested any attacks that were easy to test. That varies a little by weapon, but it typically includes left click, right click, roll+left click, roll+right click, and the Q special. The roll+click attacks are done by rolling and then pressing an attack button during the roll, which for most weapons, causes the player to exit the roll with a different attack from the one that is normally first in the combo for a simple click. As weapons commonly have several Q specials, and you can only load one for a given hunt, I generally only tested one such special. In some cases, it was the one I liked best. In others, it was whatever was easiest to test. I'll give details for each weapon below.
If a simple attack didn't interrupt a behemoth, I'd try pressing the same button several times in a row. Combos commonly make it so that clicking the same button repeatedly cycles through several attacks. I was surprised to see that in several cases, the first attack in the chain did not interrupt the behemoth, but the second one did. When I observed this behavior, I tested it repeatedly to verify that this really is what happens. If the first attack in a combo will interrupt by itself, I didn't test to see whether later attacks would also interrupt. If the first attack does, then you don't need an interrupt from later ones.
When the first attack in a chain didn't interrupt but the second did, I went to Embermane to test that same attack. In all cases, it turned out that the first attack in the chain could interrupt Embermane, but could not interrupt Skarn. That means that we have the weird situation that which attacks count as "heavy" to interrupt actually varies by behemoth. Regardless, the results below are generally for Skarn unless stated otherwise.
All testing was done without getting weighted strikes up to +6, which would allow some additional attacks to also interrupt. I don't have an easy way to get weighted strikes to +6.
Comments
Most ways to stab a behemoth will interrupt it. That includes a simple right click, roll+left click, and roll+right click. The ranged attack from the usual Q special of valiant overdrive will not interrupt.
A simple left click will not interrupt Skarn on its own, but the second one in the combo will. The first left click will reliably interrupt Embermane.
Axe:
Everything I tested interrupts the behemoth. That includes left click, right click, roll+left click, roll+right click, and the Q special grim onslaught. I did not test the default Q special of flight of ruin, but I like grim onslaught a lot better.
Hammer:
Every way to hit the behemoth with the hammer itself interrupts it. That includes left click, roll+left click, and forward+left click. For hammers, the forward attack is a different attack than the stationary attack, but either attack with a hammer will interrupt. I did not test the right click options of hammer jump, double jump, or evasive blast, as they are for movement, not damage.
The Q special concussive salvo does not interrupt the behemoth, in spite of the tooltip explicitly saying that it does. That surprised me, so I tried several times and could not interrupt Skarn with it. It's possible that I was missing, as I didn't see damage numbers appear. The accuracy on concussive salvo so awful that you're likely to miss the broad side of a barn even from point blank range. Regardless, if concussive salvo has shorter range than a basic hammer attack, while also dealing far less damage and making you fuss with reloading ammo, that's pretty useless for interrupts. It's also pretty useless for everything else.
Chain blades:
Most chain blade attacks will not interrupt. In particular, simple left click, right click, roll+left click, or roll+right click will not interrupt Skarn. The Q special of reaper's dance is movement only, not damage. I did not test the Q special cruel riftstrike because it is awful. Using reaper's dance followed by a simple left click attack will deal more damage in less time with a more predictable location than cruel riftstrike alone.
The second consecutive left click, on the other hand, will interrupt Skarn, even though the first one doesn't. The first left click will reliably interrupt Embermane, however.
War pike:
Left click attacks categorically do not interrupt. That includes a simple left click, the whole left click combo, and roll+left click. It also includes pike charge, which sure looks like a heavy attack, but would probably make interrupting too easy if it could interrupt.
The Q special concussive payload also does not interrupt, in spite of the tooltip explicitly saying that it does. I tried several times with low ammo quality and could not get it to interrupt. Neither a quick blast nor a longer, properly timed aim would interrupt. I didn't test higher quality ammo. But that's okay, as against behemoths that are possible but non-trivial to interrupt, rooting yourself in place for an extended period of time in an attempt at interrupting is a death wish, anyway.
That leaves right click attacks as the way to interrupt with a war pike, and this is where it gets weird. Right click attacks spin the war pike around for a long attack, potentially hitting a behemoth multiple times in a single attack. The first attack in the right click combo does not interrupt Skarn on any of its spins. The second attack does. The attacks take a long time and move you, making a war pike interrupt difficult to pull off.
Against Embermane, the first right click attack can interrupt, but only the very end of the attack. The animation shows the spinning war pike pass in front of the player three times over the course of a single attack. The first two can deal damage, but will not interrupt Embermane. The third one at the very tail of the attack will reliably interrupt Embermane, if you can hit it with that part of the attack rather than the two earlier passes.
Against Skarn, roll+right click will sometimes interrupt and sometimes not. Even with the proper animation to show that the roll+right click combo worked, it sometimes interrupts and sometimes not. Meanwhile, roll+right click will interrupt Embermane. However, this uses a lot of stamina for the roll, in addition to the attack taking a long time, so while it will interrupt if you can land it, this is difficult to do. The third pass of a single right click attack comes a lot faster, making it easier to time an interrupt that way.
The upshot is that while a war pike can interrupt, it doesn't have any remotely reliable way to do so, as everything that can interrupt is pretty difficult to pull off. If your strategy against a particular behemoth relies heavily on interrupts, then you shouldn't use a war pike against that behemoth. All other weapons in the game except for ostian repeaters are much, much easier to land interrupts with.
Aether strikers:
The simple version is that a left click will not interrupt, but a right click will. Putting a roll in front of it makes no difference, as roll+left click does not interrupt, but roll+right click does. Going through the full left click combo will not interrupt, either. The Q attack of surge does not interrupt. I didn't stack all of the mantras to see if adamant bolt would interrupt, as that's much harder to do. But if a simple right click will interrupt, you don't need anything more complicated.
Ostian repeaters:
Ostian repeaters do not interrupt. Neither a left click nor a right click attack will interrupt. Making them empowered still will not interrupt.
The tooltip says that an empowered saboteur's grip mine that is left to charge for a while can interrupt. I'm not sure if I believe it, considering that the other tooltips about Q specials interrupting are wrong. I didn't test it, but it sounds so hard to do that if you need interrupts, you shouldn't use ostian repeaters.
I've personally been using it. All you need to do is Aether-charge it, then interrupt by tossing it in the air when the Behemoth is ready for booping.
This is not a game.