Pick up Orianna. She can carry any game at low/mid elo, and here is how.
Step 1: win your lane with ball control and superior trading ability via E + auto attacks. Orianna is innately tanky and hard to trade with. She was designed this way to encourage people to close and not just hang back at 850 range spamming QW. If you take E at level 1 and simply walk up for 2 auto attacks, shielding the one spell they can return fire with, i swear you will win the trade versus everyone except an Annie who went W and charged stun in base. Just make sure not to dive into the creeps.
Step 2: ward and countergank. Orianna is all about battlefield control. If you see the enemy mid heading towards bot, follow through your jungle and ping your jungler to show up as well. Orianna thrives in large fights; she is less effective in 2v2 or imbalanced fights due to the ease of enemies splitting up when there are fewer of them or they can just kill you with superior damage. A good ultimate that hits every enemy in a fight will pretty much 100% ensure a victory, it helps your whole team do massive AoE damage. Suddenly the Caitlyn is also an AoE caster when Q hits 3-4 champions, and Taric drops 300 magic damage + armor shred on the whole enemy team.
Step 3: farm like a madman. Orianna is probably one of the safest farming CHAMPIONS, not just mages, in the game, due to her W and her passive. Her passive makes last hitting a breeze, so you have plenty of time to choose when you want to walk to a dangerous position to last hit a far-back creep. This makes it harder for the enemy to predict when they can harass you. Your W makes you ungankable assuming you didn't blow your flash on something silly. If the enemy mid or jungler flashes, you flash, but otherwise E Q in the direction you are fleeing W is a 100% escape every time, and if that doesnt work because they used a surprise engage like Fiddle ult or WW flash ult (which you should have warded against, knowing that it is a very strong gank but requires level 6 to pull off) you can always Q R and pull them away from yourself. Seriously, ganking Ori is the most challenging thing besides ganking LeBlanc (lol, impossible).
Step 4: repeat step 2, but follow up with tower pushing..
Step 5: siege. Ori has strong poke, stronger pickoff, and even stronger counterengage. My most classic example of this was accidentally eating a Light Binding from a support Lux while too far forward at a mid lane tower siege. The enemy team converged, only to eat Q R W and instantly all drop to half, plus be piled on top of eachother for my team to clean up. You do not beat Orianna at either her tower or yours. The only way to beat her is to catch her out or just pick so much hard engage (Olaf mainly, this guy ruins Orianna's day soooo hard) that she cant find a target worth engaging on.
Step 6: game should be over.