The Easiest Way To Beat Lavos Is To Complete All Quests, Advance All Characters To Level 57-Up (This Is To Learn All Their Best Technics), Obtain Magus As Your Friend, Equip All Their Best Armor And Weapons And Have 99 Mid-Ethers.

Stage 1 (The Outer Shell)
Use Magus, Chrono, And Robo/Lucca As Your Starting Fighters, Keep Casting Dark Matter,Luminare And Shock/Flare On All The Forms Of Lavos (Remember To Use Robo's Healing Technics) Until The Form Of Magus Comes Out (You Will Know The Forms Changed Because A Note Will Appear Saying "Form Change" And The Background Will Change) Change Robo/Lucca (There Will Be Pauses Between Forms To Heal, Equip And Switch Characters) Into Frog Be Sure Frog Is Equipped With The Masamune And When Lavos Attacks With Fire2, Ice2, Lightning2 Or Dark Bomb Attack Him With Frog's Normal Attack First Before Hitting Him With Luminare And Dark Matter Until He Changes Form, Switch Back Frog To Robo/Lucca And Do The Same Until Stage 1 Dies. When You Beat Stage 1 Of Lavos You A Hole Will Appear Where Lavos' Head Was. You Can Chose To Go-on Or Go Back (I Think Its Better To Go Back And Restock Your Mid-Ethers And Heal {Don't Worry Stage 1 Will Stay Dead Even If You Leave})

Stage 2 (The Hardest Form)
Again Use Magus, Chrono, And Robo Not Lucca. (You Will Need A Healer For This One) Keep Attacking With Dark Matter, Luminare And Shock. Use Robo To Heal, Chrono To Revive And Magus To Use Mid-Ethers On The Team Until You Beat Him. (After Beating Stage2 You Go Straight To Stage 3 THERE IS NO TURNING BACK After Stage 2)

Stage 3 (The Easiest Form)
For This One (Since There Is No Pause Between Stage 2 And Stage 3) You Will (Still) Be Using Magus, Chrono, And Robo The Strategy Here Is To Keep Attacking The Very Right "Lavos Bit" When You Kill It Both Lavos And The Left Bit Will Die

The Ending
You Will Go To The Ending Where Chrono's Cats Jump Into The Time Portal And Chrono's Mom Follow's The Cats Into The Time Portal And The Credits Will Roll While You Go Fly The Epoch Around Looking For Her And Meet Your Friends Along The Way.