Batman on Red Sky, Niv Lugassi, 2014.

My suggestion for Mecha-Frieza & King Cold vs. Goku in Future Trunks' timeline

The Z-Fighters (Vegeta is included here) probably ambushed on King Cold's soldiers (there were 16 of those men in the manga and 22 in the anime who IMHO were stronger than most of Frieza's forces from the previous sagas, with their leaders/elites surpassing even the Ginyu Force to make it worthy they were selected to serve the secretly/shadowy King Cold himself) who were on their way to exterminate humanity and fought them on a suppressed level so Frieza's "scouters" won't pick them (Vegeta told them Frieza and his father [without knowing who Cold actually is] must have a scouter on them so it'll be best to fight like Goku did against the Ginyu Force, with quick bursts of power without it being noticeable to the scouters, meaning they won't be registered anywhere).

After taking care of these henchman the Z-Fighters will order Bulma and Puar to stay at their place as they quietly and quickly sneak behind the spaceship. At the same time Frieza and Cold will wonder what have happened to their men as none of the soldiers has contacted them nor generated any destruction effects in the horizon. Vegeta will command on a blitz attack once hearing Frieza says that their soldiers may have ran into one or more of the Saiyan's friends and that he will take care of things on his own to amuse himself till Goku will arrive on Earth, they charge towards him and begin to power up to max, the ones who trained under King Kai uses Kaioken (if only Toriyama had a brain to realize that's the result of training under King Kai), Piccolo (laying his weighted clothes quietly on the ground), Gohan and Vegeta power up to max.

All charge with screams of amok at Mecha-Frieza (who is surprised at their presence as he could not sense ki nor knew that Krillin was resurrected, or for Piccolo to be a citizen of Earth nor of Vegeta's living there now, too) while aiming at his cybernetic body parts, Krillin & Yamcha combine Kaioken with Kamehameha, Chiatozu combines it with Dodonpa and Tien combines it with Tri-Beam, Piccolo uses Makankosappo, Gohan shoots a Masenko and Vegeta uses Final Flash. Frieza comments that his cybernetic eye indicates that each and every one of them is stronger than he was when he first arrived on Namek, and explains that he would have been in a "mess" if he was to face their current power back then. The attacks causes a huge dust cloud and the vibes of it make Bulma and Puar get blown at least 100 meters back. 

The dust clears, Mecha-Frieza is standing still inside a newly created huge crater with little to none damage to his mechanical body (minor scratches here and there), he laughs viciously and asks the Z-Fighters if they've got any more strength to try that again as "it almost tickled" him and claiming he was roughly using only about half of his true power. They are worn out, so Frieza lifts his hand and targets Krillin, sensing that Frieza's power level is rapidly growing Goku manages to lock on his ki signature from where he was in space and teleport in a split of a second with Instant Transmission to that very same spot. on the verge of the crater, much to everyone's surprise (Krillin falls on his bums with a relief), seeing this Frieza holds his fire for a second as he doesn't know if he's haunted by his past visions or whether what he sees is real, Goku turns into a Super Saiyan by his free will and asks Frieza and Cold to leave unless he'll show them no mercy, they both look amazed at each other and laugh crazily, Frieza begins to speak with contempt and target Gohan this time, Goku teleports in front of his son and blocks the blast with ease, Frieza charges up to 80% of full power (believing it'll be enough to handle him) and begins clashing with Goku, meanwhile Cold tells the Z-Fighters he'll be coming back for them once Goku is eliminated and tries to attack him from behind, teaming up with Frieza, SSJ Goku is pushed a little bit back but then uses a kiai and pushes both tyrants quite aside, Yamcha notices that Goku wasn't using his true power as of yet as he's pretty calm and sighs, Piccolo agrees but he mentions he doesn't know why Cold is grinning to see that.

Mecha-Frieza says he'll smash Goku to space dust if he'll let him power up to max, he mentions that his new body is much stronger than his biological one and that now he is able to fight "on a whole new level" comparing to how he fought on Namek, knowing he's still hadn't reached his own maximum, Goku allows him to power up. King Cold takes advantage of the distraction and quickly shoots a small ki blast targeted at the Z-Fighters, Piccolo launches a large ki blast of his own and manages to change the direction of Cold's blast towards the sky, Cold replies that Piccolo "may not be a bug as he initially estimated". Gohan asks Vegeta if he knew who Cold is and the Saiyan prince replies that he never knew of his existence, as Frieza was portraying himself as the only ruler of his empire and not as a "prince de-facto".

Goku is trading blows with Mecha-Frieza who is now at 100% and grew a bit larger in mass, while they're doing so, Frieza launches 3 death beams directed at Tien, Yamcha and Chiaotzu, Goku realizes they won't be fast enough to dodge it so he uses IT and deflects each blast. Suddenly a huge battle armor is falling on the ground, almost hitting Krillin. Turns out Cold wants to begin play hardball and asks Frieza to fight seriously once again, telling him it'll take him a jiffy to reach his true form. SSJ Goku thinks that's interesting and decides to play along with Frieza, who then again tries to aim at Krillin (Piccolo stretches his arm and pulls Krillin outta the direction of the blast). Cold is transforming into his third form and grows in power, "massively" according to Vegeta who tries to attack him while he was at it, but to no avail, Cold uses after image and grabs Vegeta by his hair, he uses only a fracture of his power and begins to pummel him in the stomach and in the face (ripping his pink "BAD MAN" shirt in the process), he then throws him down after the prince had lost conscious and blasts him with a two-fingers beam (as 3rd form Frieza did to Piccolo on Namek), Gohan is losing it and flies to catch the knocked out Vegeta before he'll be caught in Cold's blast and puts him on the ground next to one of the ship's legs, he flies up and tries to step up against Cold but to no avail and is kicked back down with force to the ground, landing into a cliff with a massive damage to his battle armor (the shoulder pads are broken), Frieza is shooting a blast at Goku's face and flies to protect Cold as he proceeds to his final form, once the transformation is over Goku is impressed and says that he is an enemy worthy of his respect as his power level rivals even his own true power (past his year of training on Yardrat), yet he acknowledges Cold is the mastermind of the genocide of trillions across the universe and decides that he will have to finish this whole ordeal as soon as possible.

Cold and Frieza fly up high in the sky, Goku realizes that they're planning to pull something nasty as Frieza did on Namek when he destroyed the planet's core, so he teleports in front of them and blasts Mecha-Frieza with full power and causes his metallic arm to melt down, he proceeds to twist Frieza's organic arm and knocks him in the chest with his knee. Cold notices this and jumps in with his full strength and pushes Goku to the side. Frieza's power is now at about 65% due to his injured biological arm, he breathes heavily with Cold protecting him with a force shield to gain more strength for the final blow. Both tyrants raise their hands and each charges a Death Ball. they proceed into combine their ki into one huge Supernova and throw the enormous sphere at Goku, who is pushed backwards he thinks about the fate of the world if he'll lose to the two psychopaths and begins to power up with rage, he launches a huge Kamehameha deflecting the Supernova upwards at both Mecha-Frieza and 100% Final Form Cold, pushing them both to the Earth's Sun killing them in the process (they begin to melt by Goku's reflecting their attack on them in addition to his own attack that was set to take their lives, and die while landing on the sun).

Power levels for this scenario:

• Frieza's shreds of a body: 0.002 (when is discovered by Cold and his crew)
• Mecha-Frieza (initial when he lands, heavily suppressed): 40,000,000
• King Cold (2nd form): 130,000,000
• King Cold's Men: 5,000 - 150,000 (varying from Nappa's level to over Captain Ginyu's)
• Vegeta: 3,000,000
• Vegeta's Full Power Final Flash: 4,500,000
• Piccolo: 2,800,000 (weighted clothing)
• Piccolo's Full Power without weighted clothes: 3,100,000
• Piccolo's Makankosappo: 4,000,000
• Son Gohan: 1,800,000
• Gohan's Masenko: 3,850,000 (Full Power)
• Krillin: 750,000
• Krillin's Kaioken (x3) + Kamehameha: 2,700,000
• Tien: 800,000
• Tien's Kaioken (x3) + Tri-Beam: 2,800,000
• Yamcha: 700,000
• Yamcha's Kaioken (x3) + Kamehameha: 2,200,000
• Chiaotzu: 500,000
• Chiaotzu with Kaioken (x3) + Dodon Ray: 1,650,000
• Bulma: 3
• Puar: 2
• Z-Fighters' combined might against a relaxed Frieza: 21,700,000
• Mecha-Frieza (80%): 128,000,000
• Mecha-Frieza (100%): 160,000,000
• Mecha-Frieza (65%): 104,000,000
• King Cold (3rd form): 200,000,000
• Vegeta's Galic Gun: 4,750,000
• Gohan's anger: 4,250,000
• Gohan's rage boost: 4,850,000
• King Cold (3rd form, 5% against Vegeta and Gohan): 10,000,000
• King Cold (Final Form): 285,000,000
• King Cold and Mecha-Frieza's combined Supernova: 389,000,000
• Son Goku: 6,000,000
• SSJ Goku: 180,000,000 (60% of true power)
• SSJ Goku: 300,000,000 (100%)
• SSJ Goku's rage boost: 390,000,000 (able to hold the Supernova for a few seconds)
• SSJ Goku's rage boost Kamehameha: 425.000.000