Melee weapons use a "combo" system now, where each hit on an enemy increases a numerical counter at the bottom right of your screen. Once you pass specific tiers, you'll gain a damage multiplier on (most) melee attacks. This combo counter (and multiplier) will reset by default if you use a heavy attack or don't land another melee attack in a certain amount of time (the timer is that shrinking white bar by the counter). There are various warframe abilities, focus schools, mods, and weapon perks that you can utilize to make gaining combo faster, last almost forever, or in few specific cases, never go away.
They changed this a few years back to make it easier to switch between your gun and melee weapon. If you wish to stay on your melee weapon and not switch to a gun when aiming, hold the switch weapon key/button until you see a little bright flash on your warframe. That will signify that you're in "melee only" mode. You can reset this by pressing the switch weapon key/button again.
If you had no problem killing them before the changes, then they still aren't that much of a threat. They did become a much larger threat for new players and had to be adjusted a few times because of that. As for the energy leech variant, it still exists but now has a more telegraphed attack where it will spawn a blue/purple bubble on you that will pop after 1-2 seconds. As long as you get out of range of the bubble, you won't get hit by the energy drain when it pops. If you're using a warframe that utilizes overguard (Frost, Rhino, Styanax, Kullervo), you have status immunity and can just ignore it alltogether.
I'm mostly a solo player so I can't say whether those specific farms are still a thing, but people still farm Helene and Hydron defense for leveling warframes and weapons. Depending on what it is you need, there likely is a specific location that people gravitate to. As for strats, that obviously depends on the mission type. I know that in the case of defense, players are so strong nowadays that enemies get obliterated quite easily, so having a Nova with negative strength will allow her to make enemies run faster to the defense target and thus die faster.
In most cases, if you're playing on the normal path (the normal difficulty star chart), most well modded weapons will work just fine. Once you start doing the higher end normal missions and the Steel Path, you'll probably find that isn't the case anymore and will have to be more selective with the weapons you use.
If you need a stronger gun:
As part of the "Angels of the Zariman" update, they introduced a new set of a weapons called "Incarnons" that transform after landing headshots/getting a high enough combo counter and using the alt fire/heavy attack button. When an incarnon weapon transforms, they change firing mode (in the case of guns) or get some new special effect (in the case of melee weapons). The original 4 incarnon weapons (rifle, pistol, shotgun and dagger) are easy to acquire and are pretty much the baseline for meta weapons.
If you're looking for an easy to get melee weapon, a highly recommended one is the Xoris, a glaive you get for free as part of "The Deadlock Protocol" quest.
Since then, there have been 3 more open worlds:
- Orb Vallis, Venus
- Cambion Drift, Deimos
- The Plains of Duviri