I Have A Tactics On Bridges That Has Worked For Me Very Well. But First Let Me Say That I Too Have Always Hated Trying To Attack Over A Bridge. It Always Puts You At A Huge Disadvantage And In The Real World I Would Never Even Attempt Anything Like This.
But Since That Wont Help You Much...i'll Tell You Something That Will.
Okay, First Things First, I'm Not Sure If You've Noticed But How Your Troops Are Formed At Your End Of The Bridge Makes A Huge Differance Regarding The Manner In Which They Cross It. You May Have Noticed That They Sometimes Come In On The Sides Leaving A Hole In The Center. Or Will Go Over Nearly Single File!!!!and Never Be Able To Get Much Done Before The Are Massacered. So Heres How To Form Up Your Attacking Troops. As Sun Tzu Said "the Truely Skilled Commander Seeks Victory...then Battle."
Archers: When Crossing A Single Bridge You Should Have At Least Four, And If You Can Spare Them, Six Archer Units. Now I Know What You're Thinking. Archers Are No Good On A Bridge Attack, Only On A Bridge Defense. But This Is Not So.
Move You Archers Into Position To The Sides Of The Bridgde And Slightly Back. Basically What You Are Looking For Is An Upside Down "v" With The Tip On The Exit Point At The Other End Of The Brideg. The Whole Of The Defending Army Will Back Off Out Of Range When They See Your Archers Coming. This Allows You To Form Up To Attack Without Rushing It Cause Your Being Shot At.
Line Them Up In A Orderly Fashion 2-3 Units In Closely Packed "perfect Square" Formations Off To The Left And Two Or Three To The Right. The Square Formation Is...i Think...the Best For Archers. Its The Best Chance You Have Of Having Them All Able To Fire At Thier Targets Fully.
Yari Ashigaru And Samurai: For A Bridge Attack You Need At Least One Unit Of Yari Samurai Or Ashigaru. Even If You Have A "perfect Army" (7 Monks,4 Archer,4 Heavy Cav, All Fully Fully Fully Upgraded Under A Rank 3-5 General.) That's Only 15 Units. And In That Last Place You Can Put Whatever You Want. In This Case Make It A Unit Of Ya Or Ys As Upgraded As You Can. If Your Enemy Has A Larger Defensive Force You May Want Two Ya Or Ys Units For This Strategy To Be Effective.
Form-up: Form Your Armies In Perfect Squares 8-10 Men Across (just As Wide As The Bridge Itself, Some Are Skinnier And Some Are Wider) And Just Slightly Back From The Bridge. Put Your Yari First And Your Attack Units In Sigle File Squares. When You Order Them To Cross...magic Happens...and If You've Done It Right Your Yari Will Cross The Bridge In Waves. And 8-10 Men Will Cover Every Inch Of The Bridge And Hold The Line Much Better. Trust Me On This One Its A Little Hard To Get The Hang Of But Once You Do Your Golden.
Now Don't Send The Rest Of Your Primary Attackers In. Group Them Together And Mover The Next Square With The Five Others Behind It Into Position At The Entrance Of The Bridge.
By This Time Your Yari Will Have Started Taking Fire From Archers But Don't Sweat It, None Of Them Were Meant To Survive Anyway. That's Right, I Said It. That Yari Unit...is A Suicide Unit. There Only Purpose Is To Draw The Enemys Powerful Hand-to-hand Defenders To The Bridge Exit So That Your Archers Can Decimate Them. This Is Why The Positioning Of Your Archers Must Be Exact. You Want The Arrows Coming In From The Side...not Not Not From Behind. The Defender Force Will Surround Your Yari On Three Sides. And With Fire Coming From The Sides You Will Mostly Be Killing Enemy Defenders. (by The Way, Don't Let Your Archers Be On Fire-at-will. You Want Them Targeted On The Sides Of The Massed Enemy Force. Not Shooting Wherever The Freaking Please) Of Course You Will Kill Some Of Your Own Men, But They Will Allow Your Archers To Send Volly After Volly Into The Enemy Defenders. As Soon As Your Yari Are Nearly Gone (depending On How Upgraded They Were) Your Archers May Have Already Exhausted Their Ammo. But Regardless, Send The Next Unit In Line In. But Just One And Move The Rest Forward.
When Your Archers Run Out Of Arrows. Most Of The Defeders H-2-h Force Will Probably Be Gone, And Those That Are Left Wont Be Much To Stand Against The Monks That Are About To Come Over The Bridge. But Remember, Send Them One Unit At A Time. So That Your Unit Waves Can Keep Up. Never Select Them All And Say "go" They'll All Be Dead In Minutes .
Once The H-2-h Force Is Gone Send Every Body After The Archers That Are Left If They're Not Already Retreating. Once The Bridge Exit Clears Up...send Cav To Chase Them Down If You Have Any. But For The Most Part Cav Will Only Clog The Bridge Up In Any Other Function.
And That's How It's Done. Personnaly, I Turn The Moral Off...always Have. I Fight To The Death, Or I Kill Every Last Enemy On The Field...and No One Retreats Once The Attack Starts. Unless Of Course They Attack, Run Around The Field For A Moment, And Then Turn And Retreat Without Even Trying To Engage Me Because They Simply Could Not Find An Opening ( Yes I've Won Many Battles In This Way.) So I'm Not Sure How Long It Would Take Them To Retreaet But I Don't Suppose They Will Stay Long When Thier Defenders Are Gone And You've Only Lost A Single Yari Unit.

With All Respect,
Shonen