Heavy Weapons Guy
A walking pillbox.
Advantages
Very well armoured. Incredible firepower at short-medium ranges.
Disadvantages
SSSSLLLLOOOOWWWW.
Health
100
Armour
300 (spawns with 150)
Typical role
Defence or offensive support.
Notable Weapons
Assault Cannon
A big chaingun. Its one weakness is the time it takes to start firing, but once it does start, its power is awesome at short ranges. Lighter classes will simply be mown down. Heavier classes won't last much longer. It is less effective at longer ranges, but still hurts. The combination of heavy damage and large ammo reserves make the assault cannon a superb area-denial weapon - few players are willing to walk into a hail of tracer fire.
MIRV Grenade
One of the deadliest weapons in the game. The initial explosion is comparable to a regular grenade, and scatters the four submunitions. These, in turn, each explode with somewhat more force than a regular grenade. A single MIRV will kill a level 3 sentry gun outright from 3 or 4 yards away. Most classes will meet a similar fate at ranges of 5-6 yards. Most experienced players will simply run for cover when one of these comes into view, which makes this an excellent area denial weapon.
Offensive Capacity
Some. His lack of speed is a big handicap, but a HWG can compensate for this with armour and firepower. In any simple brute-force contest, the HWG is king. If there is a single, well-defended bottleneck, a charge by a HWG can often crack it wide open with a combination of the Assault Cannon and MIRVs.
Defensive Capacity
Considerable. If a HWG is blocking a doorway that you need to pass through, you've got a problem. Once it starts firing, the Assault cannon will kill most lighter classes in less than half a second at short range, and a full load of ammo will last a long time.
How to fight them
In terms of pure firepower, there's not much you can do - the HWG outguns everything. Your best chance is to gang up on them, or use whatever speed advantage you have to just go around them. Like the demoman, the HWG is very vulnerable to EMP grenades.