Step 1: Play as the tank.
Step 2: Equip the flamethrower hero ability.
Step 3: Win.

Here is the build that I used to whip mission 18's ass.

Hero: Tank
Primary: Jackhammer (any high DPS weapon will do, this weapon should get you the flamethrower as soon as possible)
Secondary: Doesn't matter
Passive Skill: Adrenaline
Hero Ability: Flamethrower
Gadget: Kevlar (anything that increases your survivability is also good)

Toad is your greatest asset for this mission because he is the only source of cover you will get. Stick to him like glue. After all, you have to keep him alive to win. Hide behind Toad and wait for the first wave of three GlobeX to come to you. You should try to hide on the side with the minimum amount of GlobeX. If you got unlucky and GlobeX spawned on both sides of you (or on top of you), you may die. That's fine because it's the very beginning of the mission so you didn't get your hopes and dreams crushed. Not yet, at least.

Once GlobeX has come close enough to Toad, unleash hell on the invading GlobeX. Once you've killed enough GlobeX, you will gain access to the flamethrower. Use it immediately, here's where the fun begins! ;)

Once you have the flamethrower, things will be a lot easier for you. Here's what makes the flamethrower so great.

1. Incredible DPS
   
2. Gain armor on ability activation
   
3. Ability to gain flamethrower progress while using the flamethrower.
   
   • This effectively gives you unlimited ammo assuming you're always firing at GlobeX.

These three traits that I just outlined turn you into an near-immortal flamedozer that deletes all enemies that dare come close to you. The flamethrower is a perfectly balanced hero ability.
Thank you Justin. You should re-equip the flamethrower when one of the two things happen.

• You're out of armor.
  
• Your flamethrower is nearly out of ammo if it hasn't run out already.

That way you will always have a good amount of breathing room and firepower all the time.

While you may do a ♥♥♥♥ ton of damage and gain extra armor every four kills, you don't have free reign over the map. Leaving Toad means leaving cover and that's never a good idea. Let the GlobeX come to you so you can roast them. There is no need to take unnecessary risks. Sometimes you may just die to a ♥♥♥♥♥♥♥♥ bum rush.

The GlobeX spawns you get may be the difference between a successful 3-star completion and a hole through your monitor. So I would like to share with you may picks for what are low value scum that shouldn't startle you and high value targets which are worth resetting.

• Major
  
  • Assassins with Blur (I suggest reset if you find yourself face to face with one.)
    
  • Tanks with Ricochet
• Medium
  
  • Tanks
    
  • Commandos
    
  • Medics with Self revive
• Minor
  
  • Medics without Self revive
    
  • Assassins without Blur

• Major
  
  • Strong Magnum Medics/Assasins
    
  • Strong Sniper Assasins
    
  • Rocket Launcher Commandos (They're even more threatening if they have ammo feed.)
• Medium
  
  • Weak Sniper Assasins
    
  • Shotgun Commandos/Tanks
    
  • Grenade Launcher Commandos
    
  • Melee Assassins/Tanks
• Minor
  
  • Assault Rifle Medics
    
  • LMG Medics/Commandos
    
  • Shield Tanks
    
  • Weak Magnum Medics/Assassins

You will have around seven seconds of calm before you have to fight the first wave of GlobeX. The first wave will always spawn with three random GlobeX. Their initial assault is their strongest; if you can fend of the first wave, you'll have good odds of making it through.
GlobeX could spawn anywhere so you need to be prepared of all places they could spawn.

• Left fighter jet
  
• Missile
  
• Right fighter jet

Out of all the spawns, the missile spawn is the most dangerous because you are immediately in contact with the GlobeX and they could deny you the map's health pack. The left fighter jet spawn could give the GlobeX a prime opportunity to take the map's health pack for themselves; granted, the left fighter jet spawn is not as bad as the missile spawn. The right fighter jet spawn is the least worrisome of the spawns. The AI has a tendency of falling off and killing themselves before they get to you and you're only in risk of losing the map's ammo; this is something you won't have to worry about because you'll constantly have ammo from the flamethrower.

Once you reach the one minute mark (the time where Shadow tells you to protect Toad), two more GlobeX will join the fight against you. Here's where it gets hectic as more GlobeX could come along and ruin your day. At this point the only thing I can say to you is to focus up and lock in; you got this.

Mission 18 is a very tough mission and it is very possible that you may die while attempting to complete it. Here's a brief recap of what you need to know.

• Play as the tank.
  
• Get the flamethrower.
  
• Hide behind Toad.
  
• Kill the GlobeX before they kill you.
  
• Stay calm.

With these strategies, I hope that you will have an easier time completing this dreaded mission and get those coveted 3-stars! That will be all from me, have a good day and thank you for reading!