The Mantis is [b]nothing[/b] like a Titan. Titans are powered by miniature nuclear reactors, are programmed with A.I., and can only be piloted by a linked pilot. They have superior weaponry and any Spartan that attempts to hijack a Titan is dead. Even if they got inside, the Titan would just pull them out and crush them like a can.

The Spartan would toss a grenade inside of it instead of using it.

That would do absolutely nothing other than put the Spartan in a vulnerable position. If you meant Rodeo instead of Hijack, then it wouldn't damage them enough.

Even if the Pilot beats the Spartan, the Guardian would still decimate the Pilot.

No. The Pilot beating the Spartan is entirely situational (if they have Titans). Spartans have a very good chance of winning. With space magic, guardians are a huge threat, however they are largely uncoordinated. A fire team of 3 Guardians spawn on one end of Angel City, the other side has 3 Pilots. The pilots would quickly know the current location of the guardians, because of their supporting ships. A Pulse Blade is thrown into the middle of the guardians, and they are all immediately spotted. The guardians do not know this however, as they are unfamiliar with Titanfall tech. One of them is picked off at a distance by Kraber fire as it blows them and their ghost into pieces. The guardians will try to use the various traversal abilities to get around, but the pilots are too quick for them, as they easily maneuver away or into advantageous positions. A pilot and a guardian engage in CQC, but thanks to superior training, stats, and mobility the guardian gets a whole in his chest with EVA-8 fire. The Pilot tests his Data Knife on the ghost, killing it. By now the last guardian has enough energy to use his/her abilities. Depending on those abilities, he might be able to kill 2 of the 3 pilots, but the third will always get him. This is without Titans, with Titans the guardians would require several supers and would have to be incredibly lucky.

In a 1v1 fair fight, with both a pilot and a guardian face to face to fight, with both their supers/Titans charged and ready to go, I still think the Guardian would win. Also, if it's a self-resurrection warlock, they can revive even if their ghost is down. Guardians have faced much tougher enemies than Pilots.

Laser Shot easily kills said guardian, the Titan can now do literally anything, and kill the ghost. Legion tanks every super and fills the guardian with lead. Ronin slices them in half. Guardians have fought bigger. But most guardians couldn't fight on Venus if they tried. Guardians that have done that stuff were legends, they are the very small minority. We're talking about the average guardian/pilot here. Almost every pilot has pulled off massive battlefield achievements, that is why they were selected to be a pilot in the first place.

By the time the Pilot calls in his/her Titan, the guardian will have his/her super activated to kill the Pilot.

Warpfall or evade the super.