this build will work really fine, but remember that you are going to be the 5th person, you can´t use this build for a tank or carry, so you will need a tank, ad carry, ap carry and support.

i choose to use tank runes on marks and seals, i get the armor runes on seals and MR on marks, the best thing about it is that it´s really cheap. you may wanna get for exampel dodge instead of armor, but it will cost 820*9 instead of 205*9 and i think that armor is really nice. in glyph i use the only thing you can use for garen in glyph, ofc it is cooldown 0,65 %. and quintessence i take armor penetration, i take it bcs i will not get any in items.

in masteries I choose to get 23 in offense and 7 in defense. i choose that bcs I get more in offense then in defense, in defence its to much mana,AP and dodge that I don´t going to use, so i just get the armor, MR and heal. in offense i get allmost everything except more minion dmg, AP and the summoner spells i dont use, and even if i used ignite, i would not use it, im not in need of mana, then you maybe wonder why i get the magic penetration, i get it for the ulti to more dmg.

i start with boots of speedand 3 health pots, with that and Garens health reg you can stay in lane almost so long you want. then i go back and buy the full boots, and i choose berserker's greaves, thats the only atk speed you will get in this build. then you going to start with frozen malleti choose it just bcs i want some dmg in begining, and with frozen mallet you will also slow them and easier to get the kill. i´ll start with phageif you dont have money to the hole frozen mallet ( wich you wont) just so you still got some dmg. then i got warmogs armorso i get some health and i get it in begining just bcs i need the minion kills to max it out. then you should choose if you want sunfire capeor spirit visage, you choose if spirit visage if they are much AP, and sunfire cape if they are much AD. they nice thing about spirit visage is that it would get your hp reg insane and your heal much better, and atill 10 % cooldown. then you are going to get FoNwich will incrase your hp reg even more! and you get nice MR and speed, IF they only are AD you may change to randuin's omen, but thats only if they are really much AD, normally you dont get a whole team who is AD, and if there are 1 or more AP you should always get FoN. then you are going to buy atma's impaleror thornmailthats if they AD carry's who is feeded you should get thornmail, unless you should go onatma's impaler.

just got one thing to say, you cant really go 100 % on any item build in mobafire, bcs it will be things ingame who you maybe must change items, but i think this works really fine when it comes to a normal game. and if the other team got jungler you must buy wards, even if they dont got jungler you should buy it, but not that important.

i choose to start with judgement, bcs you can get some minions in begining and if you are some kind of ganked you are alowed 50 % less. then i make Decisive Strike and judgement 2 points before i make one in Courage, after that you may see a patterns in it. and the ulti is really nice to finish someone of, so if there are people who gets angry if you killsteals with it, don´t listen to them, ofc you should not use ulti on someone who is clear dead, but if it´s little unsure just kill him, never let someone else kills if you arent 100 % sure that he is going to die.

I KNOW! how can i use heal?!?! but really i choose it now bcs in lane you can really change a fight, if they almost kill you and your mate, you pop heal and you are killin them, and i want to use fortify but i dont like fortify if I dont put in reinforce. and with spirit visage you get your heal and hp reg much better.
I´m using exhaust to get kills in begining and lane, and one of the spells who is really usefull in midgame and lategame.

As you see i´m using the carry side in masteries, i´m using the tank side in runes and im using the support in summoner spells. so there you got all you neeed, and the items i really like, you are starting with some dmg bcs you need the kills in begining to get the items, bcs get much minion kills with garen isn the easiest thing to do.

This is the offtank Garen, I really like it and so can you if you test it. just dont write it sucks and vote negative, if you dont like it, sa to me what wrong with it. and some of my english is not the best but try to uderstand. peace!