For RvR, I will split this tier into 3 zones :

a) Ranks 21-24

At those Ranks, you can have the first mastery spell of any tree. Generally, Agonizing Torrent is bad for RvR (for now) and Seed of Chaos is "ok" but not something that will change your gameplay. So I actually propose you spec Havoc for these levels. Perils of the Warp is a really strong spell. In fact it is one of the strongest abilties any class can get at those Ranks. Just focus on applying Perils on every caster you see and using your typical CC as much as possible. At these levels you will be in Tor Anroc probably, so avoiding close range specs is wise, unless you have a reason to do it.

b) Ranks 25-28

You can now pick the second spell of any mastery tree. All three second mastery spells we get are pretty good. However Daemonology is still a weak choice for RvR and Havoc, as I have said earlier in this guide, does not shine until much later. So, in my opinion, best thing to do at those Ranks is to focus on changing. Indigo Fire of Change is actually your best DPS at those ranks and Endless Pandemonium is a pretty nice tactic. Just remember that, while it can make you top the DPS easily, just spamming DoTs is not really a good option as a Magus. You should use Glean Magic + Pandemonium + (perhaps) Seed of Chaos on a bunch of enemies and then focus on interrupting, knocking and dealing your single target damage through Indigo Fire and Rend Winds.

c) Ranks 29-30

Could skip this last level zone to be honest and talk about it in next tier, but for a Magus playing with friends (or a guild premade or something), it is actually worth it to spec for Chaotic Rift as soon as possible. At Rank 29 you can get Warhammer Gold (just) Chaotic Rift by going full Daemonology. Then you should add all the other useful things, including Agonizing, Aegis and Daemonic Pact as you continue leveling. The idea behind RvR with Chaotic Rift is the following. Focus on survivability stats (Wounds, Toughness, Initiative) and abilities (Misdirection, Aegis) as much as possible. Get a dedicated healer and if possible a tank to guard you as well. Move in, towards enemy caster group and make sure you catch most of the healers otherwise it is a bad rift. Try to keep enemies there (that Tank friend can actually help a lot) and help the Sorcerers and Marauders AoE them to death. You will die a lot at first tries until your party learns how to do it but once you learn you will probably love it.

IV) Tier 4

For PvE nothing really changes. Daemonology remains the best choice and at 32 you will be able to spec like this. You can actually use CR to aggro mobs and close quarters increases your damage done a lot.

For RvR, by now various specs and tactics are viable so I am not going to go into details in this section of the guide. After 35 I find all 3 Magus' specs to be nice, with Daemonology being the best for premades for sure due to Rift.

Buy  Warhammer Gold for your character to get a higher level for your character in short time.