There's a novel to be written on these game modes but I'll strictly cover tips for running the taco here.
The most important advice I can give is to choose a fast character to run the taco. Secondly, DON'T engage every enemy you see. Learn to pick your fights and predict where the taco will spawn. If you haven't caught on yet, the taco spawns where the least amount of players are and it never spawns in the same place twice. Considering the taco only spawns in three locations per map, if you can observe everyone's locations, it's EASY to predict 70%+ of the spawns.
I play with any fast characters and use the speed boost. My favorites are Twilight Chomper, Roadie or Mechanic, Toxic Brains, Rugby Star, and Alien Flower.
If you find yourself outnumbered, sabotage the opponent just near their base instead of engaging in obvious areas (this goes for all characters). Play your abilities and health down to the last second and take advantage of the life replenishment that comes with grabbing the taco :
I wanted to show the advantages of Twilight Chomper with burrow enabled (very few people take advantage of this playstyle) so I'll give a little advice on this specific character and capturing/running the taco:
It's imperative not to digest anyone while you have the taco. I'd rather goo/chomp someone and not swallow them so as to keep my speed/avoid the digestion cool down. Before you grab a taco, try to set spikeweeds on your exit pathways and goo anyone in sight.
After you capture a flag, the majority of the time, your abilities will have regenerated during the taco run. DON'T try to kill everyone. Many times, you'll have someone dead to rights and your team can finish them off, if you catch them in spikeweeds/goo so resist the temptation to spam abilities; they're tools/keys to successful taco running! Being frag-greedy is horrible for capping tacos and camping the expected/incoming spawns is imperative.
As soon as you capture a taco (map/taco-spawn location dependent), burrow back to the next spawning taco and save your abilities until just before you grab the taco. Sometimes, the enemy will beat you to the taco and instead of trying to kamikaze into the taco/race your opponent, slow down and let them grab the taco so their abilities will be neutralized.
Lastly is the running itself. When you have the taco, try to run in a random line, if enemies have you in their sights. Duck behind rocks, buildings, etc, anything that can obstruct someone's aim/incoming bullets and don't give up regardless how low your health gets.
Much of my advice can be seen in these videos:
I hope that helps someone... I'm sure I'm forgetting tons.
