PAC-MAN MUSEUM+

Pac-Man 256 missions are going to be the longest grind of all the pac-man games. It may be wise to do these before worrying about playing 765 games or earning 7,650 coins as you'll be making some decent progress to that already here. For this achievement you'll want to separate your playtime into skill play and farm play. There's a lot of information to talk about, but I'll try to keep it neatly organized.

Skill Play

Missions

• score up to 20k
• eat 15 fruit in a game
• eat 50 ghosts in a game
• get 256 combo

Ghosts and fruits should come with getting a 20k score, so long as you're not ignoring them. The 256 combo is easier to get in earlier game. A combo is how many dots you eat consecutively without any break. Your combo ends should you: turn around, stop, hit a dead end (no dots in the area), or go the wrong way on a speed boost.
The main thing you want to be doing while playing this game is getting into the habit of looking ahead in the map, and planning where to go before you get there. Keep in mind though, staying alive should always be above your combo as you only have 1 life and cannot get more in this game.

Ghosts

In order to even have a chance at getting to 20k, it is imperative that you understand how the ghosts work. Understanding this will increase your survival time immensely. Note: Most ghosts can't go the wrong way on speed boosts and both pac-man and ghosts will get brief immunity should either go through a side path.

Red: Chases you via the shortest path. Faster than you on straight paths but slower to zig-zagging.
Grey: Sleeps but chases you for some time before falling asleep again when near.
Glitchy: Chases you a little less aggressively but can teleport.
Blue: Prioritizes circling walls in horizontal movement
Orange: Prioritizes downward vertical movement
Pink: very fast single lane, wall-to-wall movement, but won't move until you enter its lane
Green: A group travels back-and-forth down a lane without stopping
Purple: A slow group predicts where you'll be headed and moves in that direction to cut you off.

Scoring

For more advanced information on how scoring works in this game, expand this section's spoiler tag, but the tl;dr version is eat fruits, get combos, and abuse powerups.

Farm Play

Missions

• earn 50 coins
• defeat 100 ghosts
• get up to 10 power ups to level 8

Most likely from going for the 20k score mission you have already defeated 100 ghosts and earned 50 coins. Leveling up 10 powerups to 8 is a daunting task and from now on, we are no longer concerned about fruits, dots, combos, or score unless otherwise told by quests.
Focus now on completing quests as efficiently as possible as well as picking up every coin on the map we can get to. Regular coins are worth 4 coins and orange ones are 10.
These missions aren't challenging but expand this section for more efficiency tips:

Here is my strategy for getting 20,000 points. Which was the most difficult mission IMO. The rest are just time consuming. There of course is some RNG invoked but I was able to get a score of 26,905 with the following:

These were the 3 power ups I used

Laser - Level 1
Fire - Level 1
Tornado - Level 8

At Level 8, tornado lasts 10 or 11 seconds and gives a base score of 35 points per ghost killed and it has a huge kill radius. Try to have a fruit active when you use it as well. This racks up the points very quickly. You can level up the others, but I strongly recommend getting tornado to 8 first. It takes 4,078 what I call "pac points" ( the points you get from picking up the yellow and orange coins on the playfield ) to get to level 8 so plan very carefully if you decide to Level up other powerups too.

It also helps to know the patterns of each color ghost

Blue - Turns left at every opportunity
Orange - Moves down til it hits a wall then turns toward you
Red - Pretty much turns toward you whenever it can
Grey/Black - Slepps til you get near it then chases you
Pink - Moves in a straight line when you cross it. It is also the only color that moves faster than you
Purple - Moves left and right slowly depending on where you are in relation to them.
Green - Just moves side to side slightly slower than you

Note 1: A power pellet can reverse the movement pattern of all ghosts except green and purple.

Note 2: When a power pellet is active the pink and grey/black ghosts will not activate.

I hope this helps

The 20K Powerup Guide

(Tornado got Nerfed so I have an update for this)
After Nerf*
Beam lvl 8
Thunder lvl 8
Twin-Nado lvl 8

Powerups to use that are OP:

Tornado lvl 8
Twin-Nado lvl 8
Laser or Better Derivative Lvl 8

Was able to get to 40k relatively easily with this combination the other guides have some good fundamentals but if you want to know the best powerups these are the best by far early to mid game.