Depends on situation really. Any weapon with high penetration is a way to go, if its on the wall situation, then you can engage with supports in melee, its a fair fight until they bring fist. But from my own experience, nothing hardcounters JPA as simply being aware of their presence, element of surprise is their most powerful weapon, strip that away and you have just GA who can bail on demand.
That's an easy and somewhat cunning answer . Buff the cap class in the short term. I've said that since about day 1 though, or day whatever I registered. Also it should be noted, you'll often get the kills around the cap points, if you've got about 2 buddies even more so. Medium term yes, buffs for territories are a bonus for the meta to matter. Only buffs that don't mean one side just gets stronger, and stronger, but instead gain more utility would make the most sense. Long term, well, let's not open that can of worms in another thread.
Speaking as someone who both uses his Stormboy solo and actively tries to counter enemy JPA troops when part of a pre-made.... -At long range Devastator equivalents counter JPA's. -At close range ground assault equivalent classes counter JPA's. -At close range Supports stalemate JPA's A JPA normally has to drop between 150 and 300 lp on fuel upgrades for his jetpack. Without those upgrades he is faced with either walking into melee range, or being incapable of dodging once in melee. Trying either of those things is a good way for a squishy JPA to die. Ground Assaults get to spend those extra 150-300lp on making themselves better in melee, so in general they will have better weapons/armour/mods and simply out-class the JPA. Supports tend to have equal combat capabilities as a JPA, with the addition of a number of special abilities that let them outlast the brief window JPA have to kill someone before their jetpack draws the attention of every surrounding enemy. FYI every time a JPA jumps/slams/dodges/ leaps through a window/leaps up a ledge he shows up on the minimap. Because of this JPA's are always racing against time once they engage, so supports are a pita. If you get jumped from behind by a JPA, it means you were not paying attention to your minimap/ears. We shine out like a giant red beacon any time we do anything more than footslog and have a very distinctive jetpack noise that drowns out everything else.
I play much JPA archetype, and there's something players do to "gain" time against JPs actually helping winning almost any encounter. They roll. Sure the roll is nice, you're trying to put some ground between you and the JP, but well if the JP saw it coming, he can just do a strong attack and pick you at the end of your roll. Or he can pistol shot your face to death. Or he can use a fuel charge + attack at the end of your roll. Just try to fight JPs with appropriate melee moves, at least you make them loose time. Sometimes you'll die, sometimes you won't, but at least you take your chance, and it's far more chances than trying to dodge roll. And if you absolutely want to roll, at least try to do it to "block" the JPA behind an object, I assume most will derp clang on the object because of lock-on. But really rolling backwards after a JPA hit you once is probably the worst move you can do, especially if you're in 1v1 vs said JP.
This might dumb down the game a bit for some but in game modes where your side can take points (meaning always for offense, and only on maps where you can recap lost points on defense) I wish if there was no tactical in your squad the game would just say "your squad can't capture points, consider switching to [insert capture class name(s) here]"
This is just.......not true. Chaos spams the following units depending on map and Chaos has a lot of perks making them very powerful in melee. Traitor Assaults (This is their main cap class) Sorcerers Autocannons The reason they are "always" spamming fast attack is because most of them wield an axe or a mace. Heavy attack with those have such a horrific windup that it usually gets you killed against an aware opponent. Orks spam the following units depending on map. They do have sluggas, but generally they aren't that good until rank 3-4. Shoota Painboy Loota