A patch near the end of 2016 changed nearly all of Twinblast's abilities, changing the way he should be built and played. Apparently nobody bothered to update their guides to reflect those changes, so I'm writing this guide to remedy this issue.
Post-rework Twinblast has multiple damaging abilities with strong power ratios, but one of those abilities scales with Crit and he has no Basic Attack steroid. This gives him a lower DPS ceiling, but he can outperform other ranged ADC's in the Early and Midgame.
This guide is primarily directed at new players. If you're already familiar with Twinblast, you likely use different cards and perhaps skill him differently. There's plenty of room for variation (and some cards that are flat-out better if you manage to get your hands on them) as long as you stick with the standard ADC power + attack speed + crit focus.
http://para.gg/zad2bnm2fsj if the card section is blank. There's room in that deck for a couple more upgraded items depending on what you have or prefer.
Everything save for the Madstone Gem (and maybe Blast Harness, but that's also a Common) should be in your starting set of cards. If you don't have a Madstone Gem, craft one ASAP. It's a phenomenal starting item if you're not going to run with one of the Wards.
Your abilities give you a versatility and damage output advantage over other ranged ADC's. I abuse this by grabbing 102 Power early on. The damage output and range from this catches a lot of people off guard.
I then grab Attack Speed via Blast Harness, then focus Crit for the lategame spike. Twinblast's power advantage usually peaks right around that ugly point where I have to farm 8 CP, so that plateau usually doesn't last very long. If you have a more Kinetics, add those and a cheap attack speed item to this deck to patch over the only real feels-bad moment in this card build. Just discard it when that will give you the 12 CP you need to max out Blast Harness.
If you take the above route and use something like Major Kinetics, then your holdover item is going to be better than the Blast Harness until you've got at least a couple of Wounds into your first Spear. Adjust accordingly.
Fist of Kings as the last item gives the best multiplicative damage. I take Basic or Ability Armor while I farm that last 10 CP, or possibly much earlier in the build if an enemy hero is going nuts.
This build ends with an 84% crit chance @ 400 damage a pop.
One point in each ability ASAP. My preferred priority after that is R-RMB-Q-E. An argument can be made for R-Q-RMB-E if you carefully manage your mana. A maxed-out E still takes an eternity to CD (12 seconds), so I max it last.
RMB is Rapid Fire. It's 3 quick Basics with reduced base damage (90/105/120/135% if you hit all 3, always 135% on minions). I spam this on CD whenever I can hit a hero or tower. I almost never use it on minions. Always (ALWAYS) use RMB on towers.
Q is Grenade. You can store up to 3 of these, and they come out pretty quickly. They hit in an AOE after a delay that's ability-reactible but otherwise too short to dodge. They have terrible base damage for their mana cost, but they're an invaluable tool for Twinblast. They significantly outrange your basic attack, don't slow you down, hit in an AOE, and give you solid between-shot damage when you use all 3.
E is Rocket Dash. This is a really (REALLY) short dash on a very long CD with a very high mana cost. It's still really versatile if you use it right. It will give you enough space to escape most melee characters and can dodge most abilities on reaction. It's not great for chasing and might instead be worth saving in case you get counter-ganked.
R is Ventilate. This is a high-damage DOT beam for 3 seconds, during which you can move at full speed but do nothing else. It has a 3.0 power ratio but doesn't scale with Crit or Attack Speed. Its deceptively long range and DOT nature make it an incredible chasing and finishing tool, but Basics-Q-RMB (and eventually Basics alone) will outdamage it. It is hitscan, so it can sometimes be useful if you expect to miss Basics.
First point can be in Q or E. Your Q is very mana-hungry and not worth spamming for harrassment early on, so IMO there's no need for a Mana Potion. I start with a Health Potion and the Madstone Gem.
Lasthitting is the single most important technique for any ADC. It generates CP's far more quickly and reliably than any other method, and Twinblast snowballs really well. I focus on landing lasthits using only Basic Attacks and the occasional single grenade. You can out-trade enemy heroes when your abilities are off CD, but savvy players will punish you if you lasthit using an ability.
Your burst potential is very high and grows over time, so try to harrass between lasthits to set up hard commits for kills. RMB is usually fast enough to follow up a Basic Attack.
Your grenade accuracy will make or break your early game. Your ganks become really strong if you land every grenade, and almost all of your early kills will end with a grenade hit from outside your attack range. Try to only "waste" them to zone an enemy into Basic Attack range.
This is when you're at your most powerful, but your next power spike is more than a dozen CP away. If you're not clearing a farming opportunity, then you should be moving towards the closest one. Farming close minions will often generate more CP than walking through the jungle for a chance at a kill. Minimize your travel time!
3 grenades should nearly wipe out a jungle camp and will do a lot of damage to a minion wave. I get Blast Harness at this point, so I prioritize killing minions even if I've been ganking well. A couple of Kinetics make a massive difference in your DPS.
Dying now has a huge opportunity cost. Dying after you spend dozens of seconds running or chasing an enemy hero is even worse. Don't leave yourself open to ganks and know your limitations during jungle chases.
Ventilate gives you the edge over most other heroes if you're doing well, as it works equally well when chasing and when running away. Look for easy opportunities to finish damaged heroes with a couple of grenades and Ventilate.
If you've been snowballing, congratulations! Go win 1v2's or even 1v3's and end the game before the enemy team hits their own power spikes.
Grenades become a lot less useful after you finish the first Spear, as aiming them can delay your Basics. The majority of your damage now comes from Basics and RMB. Ventilate becomes very situational because you'll be trading DPS for range and speed.
You are both extremely dangerous and extremely fragile. Your job is to stay safe until somebody engages and both teams commit. Death timers are ludicrously long and your team may auto-lose any fight when you're not around. Accuracy is the only thing more important than your positioning on the map and positioning during fights. No pressure.
If your team isn't coordinated but one of your teammates is doing really well, pair up with him and focus the most dangerous enemies. If your team is also losing, it's sometimes worth sacrificing your life doing this if the teammate can then clean house.
Your primary goal as Twinblast should be to abuse his strong early and midgame and snowball your way to victory. Plan B is to use your grenades and smart time management to outfarm everybody in the midgame so you can hit your Crit powerspike early enough to wreck teamfights.
If you end up in the lategame without a CP advantage, the other carries will out-DPS you but the difference isn't that large. You can still win fights for your team if you can position well and make accurate shots, and at this point a teamwipe usually ends the game.