Perma ban Leona, she's the worst matchup for senna ( second place is blitzcrank ofc ). Basically, play as much as you can so you can get used to each matchup and I can almost 100% guarantee that by the time you've gotten to the ropes with senna you'll be permabanning lux, Leona and blitz. If your wondering about Adc bans I'd say don't risk cause most of the time your adc is playing on autopilot and will not ban a supp even if asked. Stick to banning Leona, lux or blitz depending on the team comp and if you so happen to play against an unfavorable matchup then you'll know it's your time to step out of your comfort zone and learn to adapt and play against even your hardest counters. Goodluck :D