I’m not sure there’s a combination of those heroes that will do brilliantly as a defense in Platinum.
You have too little actual direct damage and direct control, and too many passive effects that can be unrolled with a dispeller.
@FraVit93’s initial suggestion has the disadvantage of being easy to unroll with a dispeller, since much of its transformation in board state is passive.
But it really uses the heroes you have to good advantage. So it’s scary enough that people who can’t counter it likely reroll. But people who can counter it will take you apart.
I would try this:
Tiburtus-Caedmon-Boldtusk-Hansel-Joon
It won’t be great. The heroes are misplaced from a maximum benefit perspective. That said, my thinking is:
The core of Caed-BT-Hansel is reasonably scary. If you attack with blue and get insufficient tiles, you’ll be damaged and mana controlled pretty fast. BT is an adequate tank, but not great. But he will reverse some damage if he fires, and few people think to bring a dispeller against just an attack buff.
Tibs in the corner isn’t great. Most fights, he’ll never fire. But late in a fight, if he does get charged, he’s going to cause a bit of havoc and could easily kill a couple of attackers. Def down late is very unpleasant, as it lets slash attacks finish wounded enemies.
Joon in the corner is bad too. At 3/70, he doesn’t really hit hard enough. But of your options, he’s the best fast mana damage. Hopefully, by the time he fires, targets will have been whittled down enough for him to actually kill.
Kiril-Caed-Cyprian-Hansel-Joon is also feasible.
There are no red dispellers, so taking red against that core is dangerous. But green is pretty easy to use against it, so wise attackers will bring Mel and Caed.
Good luck!