Senna has great damage and scaling power, she has the ability to pretty much go any build path as she scales with both AP and AD. Ideally playing with synergies but since Senna has an open build path she can fit into most team comps low elo. With a low health pool she struggles against engage supports and hook supports but with most build paths giving heavy move speed you, if you can kite you wont have a problem.

Be wary of using your R mid combat as it has a huge cast time

Your E is such a powerful move, Can be used for engage and disengage, use early mostly for ganks but rarely to catch enemies out also

Q applies on hit, Stacks Cleaver and Grasp.

Early lane phase is standard for most senna games, look to poke enemy adc and stack passive.

Look to meet these averages for Passive stacks: (Good Performance)
10 min - 20-30 souls
15 min - 40-50 souls
20 min - 60-70 souls
25 min - 80-100 souls
30 min - 110-120 souls
35 min - 130-140 souls
>40 min - 160+ souls

Early is slow for gaining souls, where as it picks up mid to late game which is why the averages aren't consistent

Her AA's apply 2 stacks of Cleaver because with her passive it applies as on hit, so if you build Aery, her AA -> Q combo will apply all 5 stacks of cleaver, lowered to 4 without Aery

Wards and Ally units are your bestfriend, as you can target them with your Q, looks to poke by bouncing your Q off the wards or allies as well as finishing low health opponents.

While taking objectives, throw Q's into your team, keep them healthy

If playing mid lane, look for souls from camps your JG takes, Most camps give 2-3 souls everytime, Red/Blue buff camps drop 1, epic monsters drop 2, and takedowns on enemies drop 1