For Errand Boy, you want to have one player acting as courier (juggling both boxes) and one acting as escort. Crouching and taking single shots with the assault rifle gives the best results; it makes you small target while granting increased accuracy. Headshots are instant kills. The escort should move up and focus on clearing out the enemies ahead. The courier should focus on moving the boxes about 10 seconds ahead of each other, leaving a 10-15 second window should things get complicated. The courier should only try to fight when they are under fire and cannot take cover.
1) Be careful going around corners. Try to only expose yourself to one enemy at a time.
2) Fight enemies from as far away as possible. Even if they look out of range, you can still get a headshot with the assault rifle at any range.
3) Keep moving. If you linger, the game will continually spawn enemies ahead and behind you, making life difficult.
4) Focus on enemies with SMGs and rifles first. Shotguns do scratch damage at best at range.
5) Watch your health. You can regenerate 1/4 of your health, and you'll be relying on that to stay alive. As soon as it starts beeping, try to hide until it refills.
6) You have a shared pool of lives. Don't get killed recklessly.
7) If one player loses their last life, restart. You won't get credit for the level if only one player makes it.
8) Watch your fire. Friendly fire is on, and nothing sucks like accidentally blowing away a teammate.
9) The escort will spend most of their time crouched. Only stand up when moving ahead and no enemies are present.
10) The escort should be using their rifle exclusively. Make sure to pick up ammo from any fallen rifle-toting enemies.
If the escort dies, the courier should focus on juggling the boxes while staying on one place. Getting involved in a fight means the boxes could reset or the courier could die. If the courier dies, the escort should take over juggling boxes until the courier gets back into position.
If you play carefully enough, you can complete the level without anyone dying.