Torchlight III

No real trick that I found for this one. Just get your first hero to level 60. Not difficulty dependent. I did not find any benefit to playing with another person except for better loot drops but the game drip feeds enough to get you through most challenges solo.

General Leveling Tips:
1. Play through the main story and you'll hit 45-46 easily without taking detours.
2. Grind the remainder in the Genie's Dungeon instances in your fort once you unlock it at the end of the story. Don't bother grinding enemies in the main story, they cap around 46 and you get vastly diminished XP gains if they are 2 or more levels weaker than you.
3. Pick up all treasure chests from quests, also Phantasmal Beast rifts, and the ones that appear before the boss encounters in genie dungeons, they give you XP.
4. Don't get too attached to Orange equipment on your level grind, make sure you are upgrading core stats like DEF and ATK. Enemies scale up quickly level by level so you need to stay on top of DPS and survivability. Do SAVE at least 1 of any "Set" equipment you obtain it'll make wearing full sets much easier later on.
5. Pick a few attacks to spec into early and get maximum bonuses for their damage. Make sure you have your Pet loaded with all 4 skill slots as well as soon as they become available. When a piece of equipment says adds "Active Skill" that is a skill that needs to be mapped to a button manually and used or it does nothing. If you don't want to deal with that, look for passive skills, or skills that procc on attack / skill use. You can slot 2 "tag" type items on your pet in addition to the collar, I highly recommend one that decreases trips to town by 90%. Keep one for the duration of the game and upgrade it when you get a newer one (on 2 60 sprints so far I've found at least one on each before level 20). Having 2 of these types of items DOES NOT stack the buff, so you're free to pick another strong tag if you ever find it. If your pet likes to die, there's one that will decrease their respawn time by 90% also.
6. You cannot change your relic after you create a character so take the time to read through what they do or look up current meta guides for the game. Each class also has a particular attack that you will need to kill XXXX amount of specific enemies to get another achievement, so consider that as well when building your tree.
7. Through your fort, obviously, all equipment can be shared if applicable. You can also transfer Respectacles if you haven't applied them to a character. Money, resources, pets, and pet skill unlocks are also shared across characters. Progress in the endgame genie dungeon is character specific, as is map availability and teleporting (based on story progression).
8. I found aggro to be difficult to hold or direct meaningfully, so having a passive from a pet that spawns minions can be a great way to distract enemies. Even bosses do not focus very intently on you if you walk more than half a screen away. Walking an entire screen away when possible will usually cause most enemies to stop pursuing you if you're in a pinch. I found the normal potions to be of the most utility because of the knockback. You also need to pick up quite a lot so make sure your potions are never full for the Replenished achievement.