First of all: In my opinion he's easier solo (and that's noobs opinion).
Second: Last area is the easiest. The slime no longer deals blastcourge but instead deals damage like lava in volcanic region does. Even his supernova isn't anything that will one shot you when you have at least 150 HP.
Attack his legs, it's extremely easy to exhaust or ko him this way. It is also hard to mount him so plan your mounts carefully
In lava region set some traps on the lava geyser
And one tip for glaive users: it's very easy to exhaust or ko him with descending thurst hitting his legs and fists