Sea Salt

This actually wasn't too hard. Here's some stuff that worked for me:

The Hunter does not count! I tried him on a couple different arenas and it never counted. He does not have a place for a medal underneath the arena menu (showing which Apostle you beat it with), so I'm assuming this is intended.

Silaris (the mermaid) was by far the best Apostle for me. The fewer people you have the more damage they do, so I use only a few high damage units. My winning combo is one Hermit, and 3-4 Creatures (at the start buy The Creature and then get your Fishmen killed, then buy The Hermit at the start of Wave 2). Just make sure your Creatures are within the Hermit's shield range. Thrown torches (orange circle with X thrown by the guys with red bandannas) go through the shield though, so be careful of those (flamethrower fire is blocked, but curiously not when they use it to fly). If you're having trouble with fire enemies you can try adding a Drowned to the mix (thanks Sassy Stan). She will completely wipe out any fire attack within range, which happens to be the exact same range as The Hermit's shield, at the cost of reducing how much damage your Creatures do (if you're using Silaris).

Swordsman (and to a lesser extent dogs) can easily take out large portions of your group. Dodging them is pretty easy once you get the hang of it. When they get close they'll ready an attack (noted by the 'shwing' sound) and then they'll move in a straight line towards a unit. This line is determined when you hear the 'shwing' so all you have to do is draw a line between the swordsman and your group, and then move perpendicular to that line (ex: if the swordsman is directly to the right of your group, wait for the 'shwing' and then move up so he slides right under your units). Dogs are less deadly and are very weak. Just let them get fairly close and then move away from them. They'll pause and then lunge at you, but if you're moving away from them they'll miss pretty much every time, and are easily taken out when they're recovering from their attack.

Balistas can wipe out your entire group in one hit, but The Hermit's shield can stop it in its tracks. If you hear one fire just hold and your Hermit will pop up his shield. Even if your other units are outside the shield your Hermit will survive.

Leave all the extra money on the ground so that you can panic buy more units if you get hit hard. If you see that you got hit with fire or something and your team may go down, immediately buy another Creature. I wiped a couple times because I couldn't tell that my whole team was taking damage and by the time I noticed it was too late. The new unit spawns around where your reticle is, so just aim somewhere safe so you don't spawn him in a war zone.

If you're still having trouble (or have other tips) hit me up in the comments and I'll try to elaborate on my specific tactics (I can also make a video of going through a few waves if that would be helpful).

EDIT: A few people are reporting that they lost arena progress when they closed the game. I personally didn't run into this but I also think I technically left the game running the whole time (I have my box set to instant on so it doesn't actually close the game). Something to keep in mind.

As with Hazar Khall, Saint Traeton was my go-to for this (and pretty much the rest of the game pre-El Huervo).

The Church arena was the hardest for me by far. I tried Silaris and Ivan Dorovich III, however they just couldn't cut it. They'd make it to round 15, if they got past round 1, and then wipe out.

My ultimate strategy was to use the wave start summon to build up an army of Hermit, Drowned, and Shaman to keep my minions alive. Once I had that, I'd use the startup summon for Cultists and sacrifice them whenever I was at the bottom of the map.

Save your gold in-between levels unless you really need to summon something immediately. Clear the gold piles around where the churches are erected because new churches will make the gold disappear. Try to leave some gold around the edges of the map for later.

Drag your army up and down the map and as soon as you're past the Dagon-forsaken train, hold so your Hermit and Drowned protect your army and your Shaman start healing. If you have gold, put the reticle on the church and summon Locusts. Boom. Church goes down in a flash. Just make sure to keep your army up, because if the locusts are your last minions you'll lose the moment they all die.

For me, Saint Traeton, with his ability to sacrifice 4 cultists for a bigger monster, won this for me.

You start with 6 cultists, and want to pick 3 more, then head to the alter first thing.

Karlshaven (1) = upper right
Rotwood (2) = lower left
Hogstead (3) = directly beside spawn
Nova Ugarit (4) = lower left
Graveyard (5) = upper left

Get 2 bigger minions from the sacrifices and end the round. Take note of the enemies that spawn wave 2, and either pick cultists for another big monster, or shaman for heals.

The strategy from there is to always pick cultists for the earlier rounds, choosing either shaman if you lose them, or creature if you get more cthulu-tentacle-face minions over dragons/tentacle beamers and are facing a lot of melee axe/hammer guys.

Dragons are immune to fire, beamers hit the hardest, cthulu homies tank the hardest and deal massive damage to barricades/towers/ballista/the church in area 4.

Arena 1 ends with the dockmaster, huge ancor wielding guy, so get a creature or 2 between rounds 12-15 so he cant run at you.

Arena 2, Rotwood, was my hardest. Always pay attention to where Ballista spawn and just immediately move to evade. I didnt use a hermit but admit he could have been useful a time or two.

Arena 3 gave me no issues personally. It was open enough to deal with Anything.

For arena 4, I just bum rushed the church if the enemies didnt spawn around it. Like reaching the arch bishop in story mode, destroying the church caused all spawned enemies to explode.

Arena 5 ends in a boss fight but my cultist-gained-monsters murdered him so fast I have no tips, he died in moments.

My only real advice is use hit and run tactics, always keep healed up, try to kite enemies to the open, and never stand still around molotov throwing guys. This achievement is a slog in an otherwise frustrating but doable 100%.

I will address any questions the best I can.
Otherwise, best of luck, and as always

Happy hunting (>*_*)>