I’ve put more hours into Mellka than I have all the other characters combined, but almost all of that was before the big patch that tweaked her venom.
I’ll just throw out some thoughts…
I feel Mellka’s helixes are pretty obvious at most levels - Spike over Lunge, every time. You can get some pretty stylish finishing hits with Lunge (flying into Dreadwind to finish Rath is my personal favorite), but most of your Lunge usages are going to be targeted at the sky, not at enemy Battleborn. As an escape, it’s perfect right out of the box. Spike, on the other hand, is pretty average from the outset, and only turns into a stellar ability through helix upgrades (+slow, +venom, +radius, +damage)
I don’t take a shield on Mellka for a couple of reasons. One, if you stay mobile, she’s the hardest Eldrid to land a crit on anyway. Two, if you take a shield item, you forfeit taking -shield items for cheap activations. Three (and most importantly to me), it doesn’t gel with how I play her. Mellka is all about applying intense pressure and then escaping in a heartbeat. She doesn’t need a shield to do this - in fact, she benefits from not having to worry about shield recharge herself. You want to be able to relentlessly bounce in and out of battle while they’re worrying about their shields.
Level 3 is actually a huge play-style decision, and worth thinking hard over. If you go with Frag Canisters, you give up the ability to lob precise canisters across the map, but your wave clear improves markedly and you can venom people more easily up close. Air Stall is fantastic against melee, but more importantly, you keep the ability to land venom on people at great distances. (If I’m not being clear: the basic canister has a much longer range than the AE helix one.) Once you get the trajectory of that canister figured out, you can freak people out by dropping venom on them from a distance well before you hit L8. A great way to harass/terrify a sniper or support that you don’t dare get physically close to.
Always use your Claw Lunge hang-time as an opportunity to toss a grenade. Spin in the air, spot a target, let loose. If you have frag grenades, aim beneath you and catch whatever you escaped from; if you have the regular canister, try to hit a stationary target. It takes practice, but landing grenades while gracefully soaring through the skies is the Mellka way. (Birds are overrated!)
I always take the right helix on five. The health stacks make a considerable difference. I also always take the seeking helix at ten. Tag with venom bullet > guaranteed finish with Blade Launcher = endgame kiling sprees. Mellka is one of the more frightening characters at L10. Not quite Orendi, but still a murder machine. Items that add to +attack speed pay off extra at level 8. At this point, though, you’re probably past needing tips. Just lay waste!
Her melee combo: use it when they’re in range, but stop meleeing the moment they aren’t - never chase in order to claw. Melee can increase your damage if you use it to extend your damage chain prior to having to reload (you preserve a few bullets to fire at them once they’re out of range, basically.) Otherwise, it lowers your DPS. Ideally, envenomate first, but keep in mind that people run when they’re venomed. Don’t chase them thinking that the melee damage bonus is worth it. Just open fire. You have a badass gun for a reason.
For all the talk of Mellka being a skirmisher and harasser, she is a legitimate assassin, once you get the hang of landing every hit with Blade Launcher. Don’t get into the trap of imagining yourself to be an annoyance and nothing more. One-on-one, Mellka has the tools, mobility, and damage to dispatch most Battleborn. You do have to be careful around CC heavy characters like Gali, but don’t Claw Lunge out prematurely - if you stick around, you may prove to be the last one standing. Also, be sure to feast on anyone who tries to perch (especially in Monuments.) Many people still don’t realize how far Mellka can lunge.
I’ll leave it at that for now, and let some other Mellka obsessives chime in