Pretty Much any Alpha egg. (for dinos) Drop about 15 in a pit to hatch then walk your dino over and start attacking from the back. for yourself, if you have a Euthanasia Gun, claim eggs, kill offspring, level fast. if no gun, drop one at a time and kill as you go. will be tough but fast xp
If I remember correctly, level 135 should be the "Max" level by default for the mod without performing any additional bonuses. However, ARK does allow fighting bosses and the "Overseer" (Depending on map) that allows a user to "Ascend". When a user Ascends, they are granted an additional 5 levels to their max cap, showing that Level 150 should be the end game goal if the user Ascends through the Gamma, Beta and Alpha Stages of Ascension.
More details are available on the official ARK wiki (listed below) regarding ascension. Please note that EC allows users to bring stronger dinos into arenas (EG: Alphas) and so these creatures can be used to fight bosses. For non "The Island" maps, I'm afraid my experience runs dry there, but I'd like to assume it's the same terms in killing bosses to level up more.