I have 12 characters above 300, right now even with no boosts i need 2 days to get one there.
The trick is always the same, skip all quest, skip all story quest, skip all maps until you are in maps around 30 levels above you, grind monsters, repeat until level 100 (about 1 hour and half max 2 if you are slow, always be buffed).
Level 100-120 fedimian suburbs grind, 120-190 demon prison 2 grind, 190 to 236 alemeth forest, 236 - 260 sekta forest, 260-300 timerys temple. This is my route, always try to mob enemies the most efficent way, make good routes and use the warpstone to optimize routes.
After you are 300+ on one character just take your time and have fun with exploration so you get the cards for all your future alts.
This is possible because they have quadruplicated exp under level 200 and balanced the exp in most zones (alemeth is giving x2 exp compared to last year).
As long as you buy pardoner buffs you will keep leveling fast, the only slow point in the game is 330-360, because most classes need gear to exp at a decent speed, and even with gear it’s about 1 level per hour of grind.