Hi Chromatic,
You will definitely be hitting the tank, and possibly more often than you may expect. Your job is to kill, assist in killing, or attempt to make irrelevant (death/make leave the fight) any target possible, with each opponent hosting a different priority, at different stages of the game. When prioritizing, you want to consider the ratio between "how problematic is this opponent" and "what will be lost during the engagement". This means you need to consider the effectiveness of the opponent (be it utility from a support causing mayhem with initiation, interrupt, sustain, etc.; be it an indomitable tank or bruiser wreaking havoc; be it a damage dealer that is mopping the floor with you or your team) vs the cost of removing them from the fight.
Say, for instance, Sejuani was constantly forcing you out, getting a clean ult on you every fight setting up the rest of her team, etc., she has clearly become a high priority. However, even if you deem her as possibly the most problematic opponent in your engagements, because of what it costs in order to remove her (damage taken, your CDs, time, loss of damage elsewhere, etc.) she may need to drop a spot or three.
Now, when you're considering what priority every opponent is, you need to do AS MUCH damage/disruption as possible to the highest possible priority, while assessing that you are as safe as needed to come out on top (as in if you can flash in and take out two opponents, you threw away safety on the assessment that your life was worth the benefit of both of theirs). This also means that you have to consider that you may be unable to hit, or do anything to, the top 3 priorities - at that point there is nothing you can change about that, and you need to do everything you can to priorities 4 and 5. If 5 is in range (maybe the support, maybe Sejuani, maybe any opponent so bad they aren't a concern), you will lay in to them until you can't anymore, or you have the ability to rationally engage a higher priority.
Note that if Sejuani is the lowest priority, but she gets down to 300hp, using the cost:benefit calculations from before, she may be then become priority 2, or even 1, OR she has remained at 5 because there isn't much opposition she can bring at this point. Also note that as soon as she is the only 1 you can deal damage to (because running past her to try to take out the enemy damage dealers means your cost to rewards reward will be heavily in their favor) she then becomes your number 1 priority, and will remain so until another engagement may become more beneficial.
Bottom line, EVERY opponent is a priority - kill all of them as you can. Lay into every opponent possible, while maintaining you stay on the higher end of the trade.
-Platinum ADC (for what little it's worth to mention)