The black spear is a powerful laning tool. In the hands of a good support it can give you the ability to make plays that will snowball you out of ELO HELL and drop you right into challenger where you know you belong. HOWEVER, this is not always the case. I usually don't like promoting distrust amongst teammates but since lets face it, odds are you won't get a semi decent support most of your games. SAVE THE BLACK SPEAR until mid game or at least until the first back if you don't feel like you can snowball off the lane extremely hard. I have literally won games because I waited until mid, made a pact with a fed squishy and used my ult to get him out dangerous situations so that he could avoid burst and deal more damage. Your ult can be both an offensive or defensive weapon and I would wait until at least mid game to make that decision. If you have a top laner that is really fed then maybe its a good idea to make a pact with him and toss his tankyness and cc into the enemy team. If your mid laner is really fed then it is probably a good idea to pact with him and use the ult to save him whenever he is caught out. Kalista is very team comp dependent and you should not just blindly pact with your support every game if you want to win consistently.

Laning phase can be a little bit tedious. You don't have a lot of waveclear and if you let the enemy push the lane it is a little bit hard to last hit due to her slow attack speed in the early game. If possible you want to push the lane so that you can poke them under their turret while they last hit. Your passive makes this extremely easy and annoying. always keep the river or tri bush "warded" with your w.

TLDR: Try to push the lane and utilize your passive to poke them under turret. If you can poke them out of lane then maybe you can get an early dragon.

Finish your items. Your playstyle will vary heavily depending on who you are soulbound to and what champs both teams have.

Against Tanky Teams: Wait for their initiation, lead them to a place where you are comfortable and fight there.

Againsty Assassin Teams: Stay near the back and ask your support to stay close. Against these types of teams its a good idea to be souldbound with another squishy to have the option of getting them out of danger. Another option would be to souldbound the tank and use your ult to pul him in whenever the assassins go for you.

Against Seige Comps: Use your souldbound with the person that has the most CC and try to walk arround and toss him from behind. The long range squishies will probably be caught out. If anyone chases you then use your dashes to dodge skillshots or chase.

Against Aoe Team Fight Comps: Always souldbound the mid laner and save your ult to get his *** to safety. Hopefully your tank line is enough to hold off the intial burst and then you can go in and get while their cooldowns are up.