After you free the Guru, you are one step closer to stopping Lavos.
Prepare for Lavos
Talk to Melchior to get the Ruby Knife. Then rest in the Inn if you wish, and save at the save point at the south end of Algetty.
Go north through Algetty and use the northeast exit to go back outside. Go southeast and use the Skyway. Go northeast and use the Land Bridge, then go northwest and use the Skyway there. Go west and enter Kajar. Talk to the sleeping Nu in the middle and say that's not Schala's pendant, and you can buy things from the Nu. (If you accidentally say that it is Schala's pendant, just go outside and back in, and talk to the Nu again.)
When you are ready, go outside, then go east and use the caves to reach Zeal Palace.
Go through north middle door and save at the save point. Go north to the Queen's room. Go north again to fight Dalton.
Dalton is easy. He only uses Iron Orb on you, which can't kill you. Volt Bite works well if Ayla is in your party. Keep your HP above 150 so that you won't die to the attack that he uses when he is defeated.
Afterward, use the Gate in the Queen's room to go to the Ocean Palace.