Explain how you manipulate their board so that "they won't be able to 1:1 senjin."
Protecting your face is not the only purpose of taunts, particularly in a Shaman deck. It's also to protect your other minions from unfavorable trades. Doomguard into Sen'jin is an unfavorable trade for the Shaman unless the Warlock discarded two cards. There have been many, many times where a Zoo player has run a Doomguard into my face and I was thankful for it. It got Hexed the next turn, I gained a tempo advantage, and I won the game. Even better is when they're forced to run Doomguard into a wolf token, totem, or taunted Squire.
What I've been arguing is pretty clear: if you're going to adjust a Shaman deck to better handle Zoo, Sen'jin Shieldmasta is not a particularly great card to put in. Wild Pyromancer and Blood Knight are. A Sen'jin is just as likely to lose you tempo as it is to gain you tempo. (Sen'jin will gain tempo if the Warlock has no efficient way to deal with it in his hand.) I'm also not saying it's a bad card against Zoo; if you have one in your deck already, you wouldn't take it out to improve your Zoo matchup. Sen'jin is a great card against "face" decks (16hp Warlock, Shockadin, Charge Hunter, etc.) It's also good against Charge-based finishers (Miracle's Leeroy, Druid's Force of Nature, and so on.) Against Zoo, it's merely decent.