Ok so the Mastodon comes with the following profile:
BS Front Side Rear HP
4 14 14 14 10
This is, in and of itself, impressive! Especially as it is a super-heavy transport so it cannot be one shot exploded by a lucky gunner. It needs to have all of those HP's ground down to zero. It has no fire points, but it can carry up to 40 regular models, though as part of this complement it can also carry up to 2 contemptor dreadnoughts with each counting as 10 models for the purpose of carrying capacity.
Additionally the Mastodon has two access points which are at the front and the rear of the vehicle.
Overview - even before you delve into the equipment and special rules of the Mastodon it has a fearsome presence on the table and this only improves the more you look at it.
The Mastodon, as one might expect, has a plethora of special rules to compliment its hulking exterior. These are Assault Vehicle, Enhanced Defensive Fire, Reinforced Shell and Void Shields (2).
Assault Vehicle - lets start with the simple one first, this vehicle is obviously designed to deliver a deadly payload and there wouldn't be much point to the bloody thing if you had to stand around scratching your arse (or ass for our West Atlantic cousins) for a turn!
Void Shields (2) - That is right! This AV14, 10HP monstrosity also has TWO AV12 void shields. Collapsed by glancing, penetrating and D hits, these can be returned on a roll of a 5+ at the end of each of your turns. Dear Throne this thing is surviviable.
Enhanced Defensive Fire - As long as one unit still occupies the Mastodon it can use its sponsons to fire overwatch, thus removing the negative of having zero fire ports. This overwatch is fired at BS2 for the standard weapons, whereas template weapons inflict D3+1 automatic hits. So its overwatch fire is better than normal! The most effective way to deal with this machine, close combat, just became a less friendly prospect... These can fire only once but at separate units as long as the unit is in their arc of fire.
Reinforced Shell - A lovely rule in a game where often bigger and badder is better. The armoured shell of the Mastodon is so secure that it contain and blast that occurs. Therefore you apply a -2 modifier to the catastrophic damage table when rolling for the Mastodon's destruction. Should a 0 or -1 occur then the Mastodon remains in play but functions as a ruined building, with both doors acting as free access points. Very fluffy and cool.
As you would expect this beast is loaded with weapons and other equipment, lets just start with the standard equipped offensive battery shall we:
Two Sponson Mounted Lascannons - The standard lascannon that can fire overwatch (only its arc of fire) at BS2.
Two Sponson Mounted Heavy Flamers - Standard Heavy flamers with D3+1 overwatch
One Fixed Forward Siege Melta Array - 12", S9, AP1, Heavy 4, Small Blast, Melta, Stone Burner
Stone Burner - if this weapon inflicts a penetrating hit against a fortification or building then it instead causes D3 penetrating hits.
One Turret-mounted Skyreaper Battery - 48", S7, AP4, Heavy 5, Twin-linked, Skyfire, Interceptor
OK so the Mastodon has pretty much everything covered here! Flamers for hordes, lascannons for armour, skyfire and interceptor for flyers and then magma death cannon for anything stupid enough to sit within 12 inches in front of its face! Blimey oh riley!
In addition to its offensive arsenal the Mastodon also has a searchlight, smoke launchers AND armoured ceramite to boot. So you not only have 10 AV14 hull points nestled behind two AV12 void shield, but it also isn't vulnerable to ranged melta!!
Optional extras with this beast include the following: 4 hunter killer missiles, an upgrade to be a super heavy command vehicle, an upgrade to have command vox relay.
Command Vox Relay - While the Mastodon is in play this allows you to add or subtract 1 to your reserve rolls. In addition any deep striking enemy units that suffer a mishap will have -1 applied to the deep strike mishap table as long as the Mastodon is in play.
Super Heavy Command Vehicle - As far as I am aware this allows re-roll of morale within 24 inches. Any one care to correct me if that is wrong?
This will be a short section as this is a pretty self explanatory! You pack it full of death and drive it at the heart of the enemy. There is no room for being timid with this UNLESS you are facing something packing shit tonne of destroyer weaponry. D weapons are pretty much the only serious threat to this other than a Primarch in close combat with retinue. But you shouldn't be letting yourself get charged really, but dictating the charges.
I would recommend the command vox relay if you have any reserve support coming in like flyers, but otherwise give it a miss. In terms of the command vehicle, that could be very very useful in 30k given that Astartes are not fearless, but again that really depends on what you have put inside of it!
So much of how you use this depends on the rest of your force that I will leave that up to you :)