Explorer notes then make rafts til level 40, learn frog saddle and tame a frog and do the first part of the swamp cave or as much as you can without dying of poison, when you get the level of scuba make several sets, and then continue swamp cave runs until max level.
There are arthropleuras level 200+ in that cave, a high health frog is necessary and multiple sets of scuba for when they break.
Alternative way is Megatherium, get to level of a megatherium saddle and kill Deathworms while under effects of explorer notes.
To increase more xp gained put a grinder by the swamp cave fill it with wood and get it going and then go into swamp cave.