Make senna an ADC OR SUPPORT
Ex: Ashe is a good definition of a support ADC Because she has nothing but her slows and ult to support her team while dishing out actual DPS
currently senna is basically an enchanter and a marksman that bursts regardless of what she builds
Passive - gains stats for every soul collected like thresh Q - bigger lucian Q while Healing Allies and doing damage W - bigger Zoe E E - Invis herself and her team R - Global instant nuke that shields her allies
are the items i see most senna's rush after 
heres how you can make her fit the support role better
Nerf the Q's bAD and increase the healing on it W is fine E should have a smaller radius Remove the shield portion and keep the global nuke
Her winrate is at a 49% rn with a 10% PR which is kinda borderline for a new released champion but itll probably climb up in the upcoming patches as people are still learning her
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She roots herself whenever she attacks.
Her E has a delay on even being able to activate, which makes it a really crappy escape tool.
R is the lowest damage global in the game.