With 170 base Special Attack, it already hits hard with its coverage moves, and will be extremely powerful if it hits twice. Unfortunately, Alakazam cannot safely switch in to Mega Kangaskhan, and Sucker Punch destroys it.
Although slightly more bulkier, it does not fare much better than Alakazam when taking hits from Mega Kangaskhan, but it can trace Mega Kangaskhan's Parental Bond.
The main problem is that Gardevoir is slower than Mega Kangaskhan, meaning it will probably get 2HKO.
Although not many Pokémon can take hits from Mega Kangaskhan, Porygon2 is bulky enough with Eviolite to take some hits, and powerful enough to retaliate hard, especially once it has traced Parental Bond.
It resists Mega Kangaskhan's STAB, and also the majority of moves commonly seen on it, although Earthquake can be a problem, he can get off a Skill Swap and Diamond Storm becomes an extremely useful move, as it has double the chance to raise Defense.
Give it a Focus Sash, Skilk Swap Parental Bond and spam powerful moves, along with Extreme Speed to counter priority users.
That's all I can think of for now. Apart from Deoxys none of these Pokémon are Uber material, Mewtwo can learn Skilk Swap but can't take hits from Alakazam, and has better things to be doing than copying Parental Bond.