Banette Plt also works wonders to use magnezone's power concistently, Ive seen hybrid's working using Shaymin Lv X (ground)'s attack to attach quite the amount of Grass energy to fuel up Lost Burn. Feraligatr prime is a common combo, Floatzel UL might work, Steelix prime could provide some assistance, and you could even try Celebi Prime.
There are plenty of combo's to use with Magnezone; but they are all decent at best. I personally would go for the Floatzel, Shaymin, or Feraligatr options;
Floatzel is a stage 1, meaning it's faster to set up, but it's power can only attach 1 time each turn, so 2 Floatzel means 100 damage, sometimes just short of KO'ing what you want. Floatzel can do this on your bench however, but beware of snipers who take out your Floatzel with one shot, so Manectric Plt might be a good choice, or some bench shields.
Shaymin is a basic, but a Lv X, meaning it becomes vulnerable while staying active, and you need a lot of energy on your hand to attach (Interviewer's Question can fix this maybe), and it wastes an attack, but if used right you could fuel your entire field for at least 3 turns of instant KO's. Shaymin is a basic, which is really fast, and if paired with the non X version of Unleashed, you could really swap around your energy.
Feraligatr Prime should be known for the combo since the first spoilers of Magnezone came out, and the only drawback of Feraligatr Prime is that it's a stage 2 just like Magnezone Prime, meaning it is slow. otherwise feraligatr is a good partner for Magnezone, but it depends on your format's speed.
Hope I helped!