Early game
You will either start out with Gem of Gaia or 4 mana pots. Former gives decent MP5 and latter one-time mana boosts. Use your skills wise and sparingly and you will barely ever run into mana problems.
If it's feasible (read: enemy is not a lvl1-burster, nor does he posess stuns or pulls) use the jungle to get BEHIND the first enemy creep wave, then walk up to the archers and dash through the entire minion group (and in case you hit a god, run further forward to lure the enemy minions off). This will usually give you a fair lead for the first minute (and usually the enemy will NOT be able to abuse your lack of escape to attack you inside of your minions).
From now on, start using auto-attacks and Combustion'd Flame Waves to farm the minions. If enemies make a forward move, try to LET THEM HIT YOU (preferably once), then stun them. Free damage dealt by your minions. Otherwise use your 3 to escape whatever non-trivial danger there may be. Don't waste it because of everything though, either Gem of Gaia or Reinforced Boots will give you the HP5 necessary to tank a few harassing AoEs.
Mid game
Starting with level 5, Agni turns from 'meh, annoying' to 'meh, annoy... DAMN YOU!'.
Save up the first 2 meteors after reaching 5, then start constantly harassing the enemy god with combustion-boosted meteors. Always ensure to hit at least one part of the minion wave with it and you will not just deal harassing damage, but as well keep the minions pushed. If necessary, drop a Flame Wave then and when.
Unless your enemy got HP-regen (or heals or buff or potions, etc), he WILL run out of hp sooner or later. This will force him to CB OR make him stay in your range. Which means his death. If you're sure you can kill him (and know there is no other enemy around / capable of attacking you), consider whether his ult will save him. If not, drop the fumes, dash into, hit the flame wave then start lobbing all meteors (keep in mind to aim ahead of the fleeing enemy (or at yourself if he decides to outburst you). Free kill. (if you didn't kill him, you shouldn't have attacked in first place.)
Keep in mind your damage as tanky Agni is not insanely huge, plan accordingly.
If you find yourself getting ganked, drop the fumes and stun the attacker. If necessary, use 3 to dash off. If the ganking enemy is squishy (f.e. a DD 4 levels below you), just burst him with your meteors.
If you built by this guide, you will often survive ganks, even if those consist of 2-3 ults. Which is effectively this build's main purpouse, wasting enemy's attention and ults.
Late game
Once you've got your Gauntlet of Thebes up, you should be pretty much immortal by now (assuming you don't rush into 1vsX situations). That's why NOW you buy the Doom Orb. Without suffering from it's drawback (no defensive stats and loss upon death) it's a great item to give you a pool of magic power. If you fill it up quickly, you may even will be able to surprise the one or other enemy with your sudden increase in damage.
To be useful in high-end teamfights, it's near always advised to grab a second power-item as last item. Since you need maximum power in just one slot, the choice should be the allmighty Rod.
During late-game, you can either keep pushing waves (utilizing your survivability and escape moves to survive teamganks) or help out in teamfights. Here it's not your damage, but the stun that counts: The AoE stun is barely miss-able and against stunned/immobile/heavyslowed enemies it's a free stunlock. Combine this with your teammate's ults and fumes become's an ultimate by itself.
(Example: Enemy are's pulls people in, you stun the stunned people again, whilst your teammate drop AoEs onto their immobile butts.)
Still keep dropping your third meteor (unless the enemy got heals) to slowly wear down the enemy hp in pre-teamfight stare-off's. You can as well initiate by dropping a meteor and casting the fumes before it's impact.
Overall, keep in mind you're a UTILITY TANKY MAGE. Draw some attention, waste skills, stun people, assist in dealing AoE damage. Don't try to 1on1 stuff that isn't terribly underleveld. Don't try to solo FG or push birds alone. Either push lanes and retreat when enemys arrive or stick to your team and pile up stuns.
Question - have you been drinking? Agni scales better with magical power than pretty much every mage in the game. Going tanky can work on any mage but I am not sure why you think magical power doesn't work on this fantastic high damage mage. Check your math.
Ao Kuang. Good bye all of your logic. But I checked the stats, and most mages abilities magic power percentage will be in-between 80 and 110, whereas Agni's Path of Flames does 60% overall, and his Flame Wave does 50% if you want to argue that Path of Flames is an ability on a tick, so it does less MP%, but some of Anubis's abilities do 140%+
The Guide is pretty good overall, I think its unfair that you have so many down votes, just because you propose an unlikely build. Upvote from me. :)
I love the idea of this guide and how it is written. I think the comical writing is what really sells it for me.
I'll be back, going to pubstomp with Tank Agni.
What does his Magical power and defense and HP end up as?
And against any smart enemy, as you said, it becomes useless, as they stack up on penetration...
Great guide, informative.