Guides are quite tricky, as no match is typical. For this reason, I have posted a sample glass cannon build I normally run with, along with some situational items I use. No build is perfect, and none are better than the others, as it depends on what you are up against!
I will instead be breaking this down into two parts : General Goals, and Item Reasonings.
General Goals
Cooldown Reduction ~30% - Cooldown is a very important aspect of Raijin, as he is a burst mage. His damage does not come from his auto-attacks like freya or chronos, or any hunter, so having your skills available sooner is critical for your damage output. (Using your skills more often means doing more damage!)
I personally seek out 30% cooldown reduction as my goal, as it is easy to obtain, and probably the best bang for your buck. By not maxing it out, you lose perhaps 1 or 2 seconds of potential cooldown on your main skills, but can gain other utilities such as more power, or more protections! (Although it is possible to obtain another 10% by some power items or protection items!)
Penetration - In order to kill objectives, you will need penetration to lower the amount of damage mitigated. In addition to objectives, penetration will also help damage those gods sporting extra tanky builds.
Magical Power (500~700) - Since Raijin is a mage, magical power is the primary source of damage scaling for skills. The more you build, the more damage you will do! While it is possible to build a mage to obtain over 1200 magic power with buffs and potions/elixirs, a more realistic, day to day build should have
at least 500-700 in my opinion.
When forming a build, you must consider : will the tradeoff of power for protections be worth it? If you are rarely taking hits, and the enemy gods don't have an easy time killing you, it would be best to go for more power than protections, as long as you can play it with minimal risk!
Item Reasonings
Shoes of Focus - These are my go-to boots for any mage. I recommend these over
Shoes of the Magi for one reason;
cooldown. While it may seem better to get another 15 power, and 10 penetration from
Shoes of the Magi, I personally feel that the 10% cooldown is a much better trade-off. You will probably be picking up a penetration item later on anyways, so this serves as both movement speed and cooldown, instead of only movement speed. The bonus 15 power is an insignificant amount to give up 10% cooldown.
Book of Thoth - A great item for mages. This item will give you a decent amount of
MP5, a ton of
mana, and most importantly, lots of
magical power. The reason why I prefer this over
Doom Orb is because this is a more reliable form of DPS, since your stacks do not reset on death.
I sometimes choose to build this item first, as it will give a substantial amount of power, and you can start building your stacks sooner. Boots can come later, as long as you save your teleport skill for mobility!
Chronos Pendant - This item is a must have for burst mages, as it serves various important roles;
cooldown, power, MP5. If you are going to be going for as much power as possible, without sacrificing your cooldown, this is the item to go for. If you feel that you are getting hit too hard by a physical god, you can replace it for a
Breastplate of valor, to trade the 75 magical power for 75 physical protections. I do not recommend this, but if you want more well rounded protections, get a
Spirit Robe instead, so you have 40 protections of each, at the cost of 10% less cooldown.
Rod of Tahuti - This item is a must-have item for all burst mages. It gives
125 magical power, 15 MP5, and a bonus 25% magical power! What more could you ask for?
Obsidian Shard - This item is needed if you are fighting against a person with high protections, or any objectives. There are other items in the game which give magical penetration, such as
Spear of the Magus, however two issues arise;
Spear of the Magus no longer gives % penetration as of the latest patch, and that you are a burst mage without skill-based auto attacks. The problem with this item is that the penetration applies after each hit with an ability. As raijin, this may not be a plausible source of penetration, because the full effect of the item may not get applied until you have already used your full kit on the opponent.
For this reason, I recommend sticking to the Obsidian Shard for penetration on Raijin.
Spear of Desolation - I like to grab this item as my last, because it will give you a greedy amount of power. If you are building to be a glass cannon mage, this item will help you get over the
1000 magical power threshold, with the help of some buffs. If your games actually last long enough that you have a full build with these items, you will be hitting like a
truck.
This however is also a good place to substitute the item for anything you may be lacking, such as protections, or other effects such as the onePolynomicon provides.
Perhaps too many random color codings? XD