I'm aggressive for a reg player, so I have a build that tries to maximise reg's lane-pushing DPS. I tend to use snipes for a bit of harrassment, but mostly for long range kills. This is a team player regulus. You do not solo with him, you assist teammates. You can deal a lot of damage. You can repel potential ganks using mine traps, but don't focus on doing that. Laying mine traps for allies to use as well as spamming mines into group battles is your key to success. Mines are your mainstay.
I find regulus seems to be somewhat more mana efficient in relation to damage output, as shrap mines drain only 770 mana maximum for 1350 minimum damage. My build is based around this.
Favor Item
If you're comfortable with reg, Staff of Renewal is probably your best bet to maximise your damage. You'll find that snipes tend to cool down 'in the knick of time' more often with this item. If you aren't comfortable with reg, you might want to choose the swift anklet.
On some maps Heaven's Wrath might be usable, but I don't seem to have a problem leveling up. BoF is another possibility if you want to be more tank-ish.
Skills
When I play reg, I find I really do well with mines. My build focuses on getting me mines-based dps as fast as possible. In group situations, mines will damage everything in range, giving you above their rated damage.
- snipe 1
- mines 1
- save
- snipe 2 + mines 2 (damage from mines jumps from 300 to 700 here, and throws small enemies - big plus)
- mark 1
- bug
- snipe 3 or mines 3 (usually mines 3 is a better bet, but snipe is allowed if it makes more sense)
- mines 3 or snipe 3
- save
- shrapnel mines + snipe 4 or mark 2 (again, get either snipe or mark depending on what you need, shrap mines is critical)
- mark 2 or snipe 4
- whatever you want (I put points into scope/maim, or go for an AA DPS build)
- "
- "
- mark 3
- whatever, etc... the game is probably over by this point anyway.
At level 12 you can choose to go with scope maim, or you can actually get most of what becomes an AA reg build, just in case the game drags on to level 20. I'll get back to this later.
Shop Items
You'll get the following items as the game allows. If you find you're getting harrassed a lot (aka, spit beast, another reg with scope), you'll probably want to focus on getting banded armor, unbreakable boots, and hauberk first. I recommend focusing on mana items, as your DPS will depend on effectively using mines as much as possible.
- banded armor->narmoth's ring (if possible)
- plate visor->vlemish->hungarlings crown
- plenor crown
- unbreakable boots
- hauberk of life
When you hit level 4-5, always carry a portal scroll or two. You'll need them for ganking or escaping.
You might need to pick up a wand of speed. I would recommend keeping a healing potion or sigil on hand. Packing mana pot is actually a pretty good idea in the early stages before you have plenor and vlemish. After you have these items, either put all your money into citadel upgrades, or into items for teammates. Getting things like a mageslayer are nice, but they won't win the game as quickly as giants.
Citadel
I usually end up having to be the first to run back to the citdel. When I do, I almost always pick up Fortified Structure 1. Usually a teammate picks up currency 1 as I often need a better mana item, such as vlemish. Later in the game, I often am the once to get cats and/or giants, as well as unit armor and damage upgrades. Sometimes I'll get currency 2 if it seems to be the most helpful.
Play Style
Teaming up: As I said, I play regulus more aggressively. I try and pair up with an Oak. A good Oak will keep you alive longer with sheild than senda can with heals IMO, because sheild will give you a few seconds to escape trouble. Basically, become a superminion for Oak. Aside from capping a nearby, undefended flag, you should support him. Other DGs you could hang around are QoT and Sedna. Failing that, hang with a tank like UB or Erebus, although a good DA can get some good ganks if he has minefields...
Early game (level 1 - level 5): Play it safe. You need to get the lvl 5 without feeding the enemy, so your first priority is to avoid death, your second is to assist your partner DG, and finally to protect whichever tower/lane you are guarding. If you can't push, don't worry. Be patient and level up, cap other flags when they are undefended and the opportunity arise. Use snipe to assist distance demis with their battles. Don't use snipe for harassment unless it's clear you'll push a demigod back. Sometimes, you can snipe a healing unit instead of a DG. If possible, kill healing nearby healing minions. This will force mana use on the generals. Sooner or later, they will run out, and either have to leave their lane or play as an assassin.
Try to carry a mana pot with you.
Mid game (level 5 - level 10): Now you can stretch out a bit. Use mines 2 to clear away enemy healing minions. Sometimes you can clear away minions while dealing 700 damage to one or more nearby enemy demis. Mark will help you by slowing an enemy and dealing some damage to everything around. You will start to focus on helping to win group encouters by casting mines into the mess. This will maximise your DPS to damaging everything in the combat area for 700 up to 1350 damage per throw.
Before entering combat, set up a mine trap along a possible escape route. If you or an ally has to run, the mine trap will often turn the tide, changing a lost battle into a dead enemy DG. Smart enemies will learn not to chase you or your allies, or at least send a minion to chase to see if any mine traps are present. You can get clever and find a way around this possibility too 
.
Use mines and AA to take down towers. Work with your lane-mate if you have one. Mines can be thrown from outside the tower's attack range, and you can sometimes damage other units if you time the throw right.
Make sure you always pack a sigil and/or health potion(s). Always carry a portal scroll.
Late Game (level 10 +): You now have shrapnel mines. It is safer for you to get close to combat now, as enemy DGs will be slowed. Keep them as slow as possible as much as possible. Keep mining during group combat. As long as you have both mana crowns, it will take a long time for you to run out of mana. Spam mines, spam mark. Snipe running DGs. Don't solo unless you are baiting for a trap and know you can win.
UB is your nemesis, so make sure he stays slow and take damage if he comes to you. Wand of speed may be necessary here. Erebus can engage you if he wants to, so make sure you have some mines around before he swarms in. Much of LE's attack power comes from minions. Mine them to clear them away. Hopefully LE will take some splash. Always throw mark in combat. LE will sometimes try to swarm to escape battle, mark will sometimes then kill him off or damage him enough for a snipe finish.
If you have to retreat from battle, which is often better to do before your health gets too low, leave mines behind you for your allies to use, and try and mark and enemy DG. Back to a safe distance, and snipe to assist. If you are not being persued, just back off far enough to allow you to continue to throw mines in. Reg does his best damage by far vs groups, remember that.
If your team is on the ball, you can use yourself as bait. In group combat, you will likely be a priority target because you can deal very good AoE damage and are relatively low health. Let oak shield you when you are under attack, and pull the other team over some mines as they hit you or throw mines and mark them as they switch targets. Keeping them slow and taking AoE damage is your best bet for winning battles.
Late, Late Game (almost never happens...)
Ok, if you end up in a game where by lvl 12 you think it will take much longer to complete, and that there is a chance you'll make it to level 20, you can switch to an AA-like build. Basically, every point from lvl 12 on goes into Angelic Fury (aka wings), and Enhanced Attributes. Once you find your regular abilities aren't quite cutting it on their own, dump the plenor battlecrown and hauberk of life and pick up the crit gloves and a mageslayer. By level 20, with these items alone you'll do over 800 aa dps, plus have all your regular abilities such as mines, mark, and snipe to soften and slow your targets. Put 1 pt into scope. At level 15 you can choose whether you want a point in mark or another point in EA, or a point in maim. As with any AA build, you need to maximise your DPS in order to pump up your lifesteal. If the games goes on this long, even an ashkandor won't be out of the question...
And that's that...
As you probably gathered, getting good with mines is key to playing this reg really well. They are a bit like rook's hammer slam, you have to anticipate what your target is going to do and try and make sure he ends up stepping on mines. Get good at mines and you will find you can deal a lot of damage very quickly.
