The Enormous Spider God was an early rumour that got a lot of people excited about the new army book. Once it arrived, most of us loved the look of the model, but once we actually got the Arachnarok Spider on to the battlefield, quite a few were disappointed with the results. Although it's got 8 wounds at toughness 6, it only has a medium armor save, no regeneration, with such a large base that allows even Hordes to fit a majority of their models in against it. Compared to the likes of the Hydra and the Hellpit Abomination, the Arachnarok has trouble matching up but will still cost you more points to field.
What's important to bear in mind when discussing these things is that the Arachnarok seems to be representing a "new era" of Warhammer Monsters that will provide key abilities and support your troops in battle against enemies, rather than take aim for the nastiest enemy unit and destroy it, like the Abomination will do to you. In fact, if you're looking for pure combat mayhem from your Monster, you're better off bringing a Giant to the battlefield.
Let us instead look at what the Arachnarok Spider will do for you, and how you can make the most out of its unique abilities. First of all, the Spider is a Goblin unit rather than an Orc one, and naturally will help the gobbos out more than it will the Orcs (in the same way that Trolls are usually more effective when inspired by a strong Orc Warboss). The most obvious use for the Spider is therefore to act as a support charger next to your big infantry blocks. Have the Gobbos, or whatever you're fielding, charge into the front of the enemy and maximize your models against one another, while the Spider hits the flank. Or, even better, charge the front alongside the infantry and clip the enemy corner to corner (since you still have to maximize and bring all your gobbos into the fight). Either way, you're minimising the amount of attacks swinging at your monster while still getting the full the combat abilities of the Spider. If your Gobbos are carrying nets, even better, as by lowering the strength of your opponents you're helping the monster out as well. With its movement of 7 and Swiftstriding ability, the Spider is actually the fastest and most mobile monster in Warhammer outside of the ones sporting wings.
The combo charge should make it possible for even weedy goblins to take out Elite troops with Great Weapons, even armored foes which they will usually struggle against. But if they're not Elites, the Spider is quite capable of taking them down on its own. In fact, the Spider should easily be able to deal with foes that Goblins prefer not to, which are large units of core troops that are better at them at fighting. Elven Spearmen, Saurus Warriors, Empire Halberdiers, are usually more scary to Gobbos than what Chaos Warriors are, since even though they will kill less Gobbos each turn, they're usually a lot more numerous meaning they remove the Goblins' ability to be steadfast a lot quicker. Such units at S4 at the most make for excellent targets for the Arachnarok, since it's got a decent weapon skill and solid toughness, should fend off most of their attacks. Even if one round of combat goes poorly, the Spider is still Stubborn meaning with good leadership nearby it should remain in combat. Inflicting 5-10 casualties each round, there is a good chance your enemy infantry will run out of models way before the Spider runs out of its eight wounds. Also, since large infantry units usually make for good bunkers for enemy mages, you might end up with juicy targets for your Venom Surge to inflict its D6 wounds with.
The Venom Surge is also why the Arachnarok will do well against enemy monsters. It will eat both Stegadons and Hydras without much trouble, especially if the Venom Surge attack wounds, and have good initiative and toughness enough to do a decent job against the Abomination (although you'll need to kill it in about two rounds - if not then it will probably have your head). The Poisoned Attacks of the Spider are also excellent ways of getting through the toughness 8 of the Tomb King Sphinxes, as long as you're careful of the Heroic Killing Blow.
The Arachnarok will benefit nothing from the Orc spells of the Big Waaagh! but will make good use of several of the Goblin ones. Chief amongst them is without a doubt Gork will fix it, forcing enemies to re-roll 6's when they try to hit or wound him, which keeps the Spider safe from Skinks when moving into battle, and will nullify some of the effectivness of Witch Elves going up against it. For the afore mentioned S3 or S4 infantries, the spell is devastating. Also good is the fittingly named Gift of the Spider God, since it makes already poisoned attacks auto-wound on 5+ instead of 6. It increases the chance of your Venom Surge hitting home when it needs to, which is always good when up against enemies where the Spider's strength of 5 feels a bit weak.
It must be mentioned, however, that some artillery will have a field day shooting at the Spider, who without any regeneration will struggle to avoid the blows. Take heart in the fact that it does take at least two cannon balls to kill it, and three on average, so with its good movement you can still be able to reach combat before they take you down. There's always that risk when fielding monsters though, but most of the times the added benefits are worth it.
Flinger: At first glance this seems like a weird upgrade, but it is one definitely worth getting. For about the same price as a fanatic, you're able to fire spiderweb at an enemy unit that make them Strike Last in combat until the start of your next turn. A cheap upgrade that you can definitely afford, and it adds an additional tactical advantage to your side. Goblin units, in particular Squigs, Trolls and to some extent Orc unit as well gain tremendously by striking ahead of the enemy - Squigs in particular. True, it is not as simple as charging in and helping your troops out in the following phases, since the Flinger can't be fired into combat, but it can give you tactical control of the battlefield. For example, by putting a sacrifical unit such as Wolf Riders between your Squig Herd and a nasty enemy, to redirect it, then fire the Flinger at the enemy, you can control the inevitable combat coming up. If it hits, you'll elect to flee with the Wolf Riders if the enemy charges, and tempt the enemy to redirect into the Squigs, fighting that combat on the turn when the enemy strikes last, or force them to stay out of combat by not redirecting and suffer another round of shooting from your artillery. If the Flinger misses, you can simply sacrifice your Wolf Riders to redirect the charge and still fire at them for another round, or charge them in your turn, if that's what you want.
Furthermore, the Flinger makes your Monster compete in all rounds of the game, much like the Treeman, which is quite rare. Although you're not buying an Arachnarok Spider to fire the Flinger alone, the more rounds and phases that your army can compete well in, the better off you'll be. This is the same reason why Magic lores such as Lore of Shadow usually needs a second lore in the army to do really well, since most of the Shadow spells get truly useful in rounds 3-6 when the armies are close together, and a lot less useful in the early rounds.
Catchweb Spidershrine: There is a huge in risk in placing your weedy goblin Great Shaman on top of your beefy monster, but the rewards are incredible, as it gives all your wizards +2 to their channeling attempts and the Loremaster ability to your Goblin. This is of course perfect for an army with some of the cheapest and, thanks to the Magic Mushroom ability, most effective wizards in the game - the Night Goblin Shamen - and if you're fielding a Spidershrine you should definitely bring plenty of them. Keeping in mind both of these abilities, it's far more effective to bring two level 1's rather than one level 2, as it gives you an additional channel and arcane item, and they should still be able to cast their low level spell with a single power dice and a mushroom. Give them a cheap arcane item each (Dispel Scroll and Channeling Staff are two of my favourites) and watch those extra channel dices add up. Keep in mind that the same channeling bonus apply for your dispel phases in the enemy turn, to the Spidershrine also helps with your magic defense. Also remember that a Loremaster ability mean you'll get a duplicate spell for each of your additional goblin levels, and there is no limit on how many of your shamans that can pick the signature spell as their own.
That being said, keeping your Shaman on top of the Arachnarok means that the Spider won't be doing as much fighting. It can still support your infantry for sure, and fight off S3 infantry without the Shaman at great risk, but stronger foes and monsters will all just kill of your main wizard and live with dying to the spider. Obviously, if you go up against cannons or stone throwers, this makes your Spider even more of a target. There are ways to combat this, naturally, and a Talisman of Preservation for that 4+ ward save is a good start. Also, a timely Potion of Toughness can be worth getting for your Shaman for that important round where you just have to charge the Spider into something nasty and pray that you'll kill it before your Shaman dies. To keep costs down (and this will be a very expensive unit for certain) it's probably worth skipping out on the level 4 upgrade for the Great Shaman, since you'll have all the spells anyway and solid enough dispel phase as it is.