Blood and Fire

Blood and Fire by Tavi is a Halo/Mass Effect Fusion Fic.

The Citadel races have long been involved in a cold-again, hot-again war with the Covenant, a technologically advanced but numerically inferior theocratic empire. When they attack Shanxi, a colony belonging to the newly encountered race known as "humanity", the Citadel comes to the rescue with offers of assistance to the newcomers, who gladly accept.

Fast forward several decades. Covenant forces attack the human colony of Eden Prime...

Tropes used in Blood and Fire include:

All Your Base Are Belong to Us: Saren sabotages the Spectre quarters.
Alternate Universe: Humanity being on much better terms with the Citadel. Garrus already a Spectre trainee. Tali on the Normandy's shakedown. Quarians and Krogan getting a better deal from the Council. Ashley is an officer and N5.
Armor-Piercing Attack: Covenant plasma weapons deal electrical and thermal damage that kinetic barriers cannot protect against.
Awesome but Impractical: The original Spartan programme was just too costly despite the extreme quality products, such as augmentations still unrivaled after 50 years. Word of God directly states that the cost of equipping and training one Spartan matches that of a mechanized company. Its replacement, the N7 Programme, is more economical without losing too much effectiveness.
Be Careful What You Wish For: The Batarians wanted to be isolated? They got just that.
Car Fu: Ashley ramming a Hunter with a hover truck.
The Cavalry: How humanity is introduced to the Citadel races via the latter doing this.
Composite Character: Shepard's origin combines several of the canon backgrounds.
Dangerously Genre Savvy: Overlapping with I Know You Know I Know, Saren anticipated where Garrus would be holding a sniper position and had his Brute retainer lying in wait.
Depleted Phlebotinum Shells: While the Covenant plasma is better in a straight fight, special effects on mass accelerator rounds have their own utility and are available even on starship guns all the way up to dreadnought size.
Doomed by Canon: Averted with Nihlus and Jenkins.
Establishing Character Moment: In-Universe for Tali. "Tali, Nihlus wouldn’t drag you across half the galaxy if you weren’t up to this. You introduced yourself to us by throwing a grenade at a crooked info-broker!"
Four-Star Badass: The Spartans we've seen so far, including John, Linda and Kelly.
Fusion Fic: The author avoids several problems with the match-up, most obviously the power level gap, by doing this. Rather than using the Covenant wholesale in their original Halo form, the Covenant here are an advanced faction with Geth- or Collector-level grasp of the Mass Effect, enabling them to keep their energy weapons without grossly overpowering the Citadel races. Similarly, the object of their religious worship is changed to the Protheans, who they consider Forerunner, rather than Halo-canon Forerunners who would utterly destroy the Reapers in a straight fight.
Genre Savvy: Garrus, when he sees Saren standing in the open by himself in the midst of the Eden Prime chaos, immediately gets suspicious and gets permission to cover Nihlus as the latter moves in.
Higher-Tech Species: The Covenant have a tech advantage over the Citadel races, with this being balanced out by personnel numbers.
Humanity Is Infectious: 'Where the hell had [Garrus] heard that? Damn human memes.'
Magic Knight: The Covenant have their own biotic combatants. Word of God directly equates Zealots with Krogan Battlemasters.
Mythology Gag: '[Jacob Keyes] made a mental note to find out who had told the cranky old Turian about air quotes. There was going to be hell to pay for that one.'
Required Secondary Powers: The Citadel races know the science behind Covenant plasma weapons, but they lack the material science and power generation knowledge to make the things work in a practical, combat-useful package rather than in the laboratory.
Title Drop: "Every turning point in [Shepard's] life has involved two things doctor. Blood and fire."