Some people are probably crying "BROKEN!"
Some people are saying "BRING IT ON! I KEEL EET!"
And some are saying "Whatevs, I'll work around it"
Teh Borken?!
Personally, I'm not convinced they're broken. Not at this point. Frankly, they're expensive (both in $$ and points) so that limits availability.
They are durable, that's for sure. Figure on ~ARM19 and about 54 boxes as a yardstick. Figure on SPD5 and reach. Other than the 50+ boxes, this is kind of like a nasty heavy with a pretty reasonable 10" threat range. This threat range (which is only extended by SPD/movement buffs applied during activation) isn't new. Thus far, the Kraken has the top threat range at 12" (SPD5, 4" melee range during activation).
Honestly, the one really novel thing that Colossals/Gargantuans bring is a concentration of power. These things hit monstrously hard and can absorb a beating. Personal opinion - Warmachine's focus mechanic gets a little more mileage out of these than Hordes fury, as 3 focus on something that hits at PS20 (or, y'know, hits a half-dozen infantry at PS15 and closethlines a jack or two in the bargain) can go a ways. On the flip side, while Colossals are doubtlessly trailed by mechanics, Gargantuans can still heal readily.
Honestly, they're just big, expensive jacks with some neat tricks and an immunity to a number of normal stopping effects (knockdown, stationary, disruption...)
BRING IT ON! I KEEL EET!
Ah, the page 5 approach...there's a certain elegant simplicity to this. People will doubtless point to the ARM19 base and 50+ boxes you'll see on a gargantuan.
On the flip side, these things are doing good to hit DEF10, and they're pretty much NEVER gonna get a DEF buff outside of a spell. What you really ought to worry about on a colossal is an ARM buff, as these things are going to takea number of hits to kill, and the more hits you tank the more you benefit from an ARM buff, y'know?
That aside, you kill regular heavies with massed weaponmasters and/or hard-hitting heavies. Given the DEF value on these things, hitting isn't hard unless your dice decide to yahtzee on one's. The most you might have to worry about is finding a way to knock off any ARM buffs on the things. The biggest difference is that you'll have to worry about making sure the thing actually gets DEAD; there WILL be repair nearby and a single focus can fuel a sweep attack and potentially ruin a fair number of those pissed-off weaponmasters. Similarly, PS20ish fists will ruin jacks they can hit, but most of these things are MAT6 base.
So, honestly, can your list deal with some heavies? If so, this thing costs about as much as 2-2.5 heavies, and takes about as much abuse. Bring a way to drop ARM buffs, then commit as many heavy hitters to it as you can to make sure it dies, because if it survives it WILL make you pay for your temerity (or bad dice). It's big, it's not going to hide. It's almost daring you to kill it.
Soft style - avoid the thing
This is also a bit of page 5, because avoiding the enemy heavy hitter is potentially another way of saying "I think I'll kill the juicy center of the army", or, y'know, you can maybe try scenario. Maybe, because colossals/gargantuans DO have staying power (as, y'know, previously mentioned).
The biggest asset of a colossal/gargantuan is it's a huge concentration of power. There's a lotta ARM, a lotta boxes/circles, and some high-powered attacks on one package. They can dish out and take a huge amount of abuse. HOWEVER, it's also just one model. It will probably get deployed centrally, and at the end of the day there's only so much room one can cover. If you have multiple threats (IE: not colossals) then you can potentially threaten the caster even if it's hiding behind the big guy.
Alternatively, anyone with Phantom Hunter might just be able to shoot THROUGH the colossal.
As another note, anyone that can cut the SPD on a colossal or otherwise inhibit its movement (IE: Locker animus, Gorgon, a bunch of medium bases it can't trample through / good placement to avoid giving it a trample landing zone) is in a potentially good spot - you don't HAVE to kill the Colossal to win.
You just have to kill the caster.
That being said, if it's a scenario about holding raw territory, a colossal is even nastier than a heavy for holding territory as it can take up a lot of real estate, AND you don't have the option of slamming/throwing it out of the way.
Wrapup
Yes, they're new. Yes, you may have to adjust your playstyle. No, they're not broken - their main defense is being able to absorb a lot of abuse while being able to dish it out again, and strategically they can take and hold real estate just about better than anything else in the game.
If you know how to kill heavies, you know how to kill colossals.
If you know how to work around heavies, you have an idea about how to work around colossals.
It's not rocket science. Back when I started this game (and when I inculcate new people) I tell them that novelty kills in this game. The first time you see something new and don't fully appreciate what it can do, there's every chance it will smoke you. That's part of the learning curve. Take a deep breath, look back at what happened, and move from there.
