Overview
-Estimated achievement difficulty: 3/10
-Offline: 12 (200)
-Online: 0
-Approximate amount of time to 200: 2-3 Hours
-Minimum number of playthroughs needed: 1
-Number of missable achievements: None
-Do cheat codes disable achievements?: No Cheats
-Does difficulty affect achievements?: No
-Glitchy achievements: None
-Unobtainable achievements: None
-Extra equipment needed?: None

Introduction
This game should be played on easy and you should save often when you are high in health. Since it's a Sega game, you can save whenever you want and exiting to load the game you saved is quick. You can do melee attacks but they are useless when you have a certain combination of weapons. This combination is that of the leaf and lightning icon pickups. All you do is hold  and it will track and kill enemies and bosses very quickly. To grab a pickup, you must hold down and press  over it. Once you have both weapon types, if not already on the blue laser that follows enemies, press  to cycle to it. 

Step 1: Beat the game
First, let's knock out an achievement for starting with each weapon time. Begin a game, pick a weapon, then once the first level loads go ahead and exit out. Do this with each weapon until you get the achievement. Afterwards, start a game for real and be sure to pick the green leaf icon one. Run through the level shooting until you find a lightning pickup. Once you have done this, make sure you're on the combination of both weapons (the tracking blue laser). Now you just run through killing everything, saving often to make sure you're in high health. You should be able to maintain 80+ all the time throughout most of the game. Your achievements will all come to you as you progress through the game. You will get the last 3 for beating the final few bosses at the end, so don't worry 

*Important*

The level where you are on a mining cart has a specific boss that changes forms. At the end of this particular level, you will want to save on file 3 when the boss warning comes up. Now go ahead and beat his first 2 forms. The third sequence will have two possible forms. One is a floating gun and the other is a giant red crab (looking) form. If you get the gun form, reload and try again to get the crab form. See picture below:

Step 2: If you Missed Crab Attack...
If you didn't create a save and didn't get the crab looking boss on the mining level, it's no big deal. Just start a new game and pick the mining stage first (the second icon over). Make sure to pick the green leaf icon, and then pick up a lightning one like usual. Make it through and you can make a save beforehand and get the achievement. No need to replay anything else.

Learn Your Tools of Destruction
Use this picture as the visual to understand each of the weapon descriptions.

In the top left is the Force Weapon. Just your standard machine gun type of weapon. Does average damage but its real power just like the other weapons is shown when combined with another.

Top Right is the Lightning Weapon. It's just like Force but it pierces through enemies so that is a major perk.

Bottom Left is the Chaser Weapon. Out of the 4 weapons it does the least damage but is the best weapon for weapon combos. It hones in to the closest thing and kills it. A good starting weapon even with it's lack of firepower.

Bottom Right is the Fire Weapon. The weapon that packs the most punch but because of this perk it doesn't have a lot of range. So unless you love close quarters stick to using this weapon in combination with another.

Combining weapons is a major game play mechanic that is important to your success. When you pick up two weapons and neither is selected, they then combine to create a whole new weapon for your arsenal.

I'm not gonna get too deep into weapon combos but if you wanna look at the chart of combination's then Strategy Wiki's Gunstar Heroes Weapon Table is for you.

Just for Achievement sake the only weapon you need is the Lightning Chaser (Lightning + Chaser). Deals with everything swiftly and all you have to worry about is defense. And in this game you always need to be ready to be on defense at anytime.

But there is one type of attack that is stronger than any gun you can acquire. That of course is Hand to Hand Combat. There are a couple of moves of this type in the game and I'll run down each one.

while Jumping: Gunstar Dive (Gunstar Kick in Fixed Shot Mode).

This is your main hand to hand move. This move can kill most bosses faster than any weapon in the game. For success in Gunstar Heroes, it is a must you learn this.

(Air) +: Gunstar Dive Kick (Fixed Shot Only).

A better version of the Gunstar Kick. Alternate between the two to see what you like better.

Close or + Any Direction: Throw.

You can throw minor enemies, a couple of bosses and bombs that are thrown at you which you can throw back at your enemies. Also weapon/health carriers can be grappled as well as shot to get more weapons/health as well. Make sure you abuse throwing in any direction. You will need it.

+: Gunstar Slide.

This should be your offensive move of choice most of the time. Also does big damage to most enemies and also to be used defensively to run if need be.

,,,,+or: Standing Gunstar Slide.

Once you learn how to do the Standing Slide consistently this is the version you should stick with it because it's leagues better than the regular.

or+: Blocking

This move is underused and underrated. When used correctly it can protect you from damage that would otherwise kill you. Abuse blocking and you won't regret it.

Conclusion
This is a very straightforward game. If you just save often while you're high in health, you should beat the whole game without dying and have all of the achievements in that one playthrough. Just make sure you always use the mentioned combination of weapons. It almost feels like a cheat of sorts because of how easy fast and easy you will blow through the game.