Tips for fighting the Ancient Sentinel

Units/Formation Strategy:

The AS's melee arm has two attacks, Crush and Pulverize.  The Crush attack, which hits the entire battlefield in a checkerboard pattern, can only target vehicles and aircraft, not infantry or critters.  It also only has a range of 1, which means it can only target the front row.  Putting only infantry or critters in the front row will prevent the AS from ever being able to use the Crush attack.  Even after your entire front row has been destroyed and the second row moves forward, the AS will still not be able to use Crush even if there are now vehicles or aircraft in the new front row.

Good front row units have high hit points and/or high dodge ability, along with attacks that work well against the AS's arms and/or body.  Legendary Boars, Veterans, Wandering Samurai, Blood Ninjas, and Elemental Troopers are some of the best units, but other units with good armor piercing, freezing, or stunning ability can also work well.

Good second and third row units include high HP, high armor units with good or at least decent armor piercing ability such as Mega Tanks, Tank Crushers, and the Wild Boar 2.  A Radio Tech is also an option.  It is weak defensively, but if you can get it to survive the first 6 rounds, its Precise Strike can do several hundred points of damage to the AS body before it's killed.

Attack Strategies:

Some people completely ignore the arms and attack only the body.  That is certainly one approach, and it can be successful if you have a lot of strong units.  The downside is that even if you eliminate the AS's Crush attack as described above, it still has 5 other attacks so you are going to get hit every single turn.  A popular strategy for those without lots of uber-units is to first freeze the ranged arm.  Then while it's frozen and susceptible to increased damage, kill it.  The Armor Piercing Shots from the Wild Boar 2 might take it out in one shot if you're a little lucky and get some critical hits, but probably it will take a least two shots.  Once the ranged arm is gone the AS loses two more of its attacks.  Now you should have some rounds where the AS can't attack because it's waiting for its attacks to cool down.  

The ranged arm can be frozen but not stunned.  The opposite is true for the melee arm.  It can only be stunned.  The body cannot be frozen or stunned so don't waste attacks trying to do that.  Stunning the melee arm occasionally will also increase the number of times the AS has to pass.  One reason the Legendary Boar is such a great unit against the AS is that it can both freeze and stun.

The only way to kill the AS is to kill the body so that is where the bulk of your attacks should be focused.  The Legendary Boar's Chilling Roar, the Wild Boar 2's Armor Piercing Shots, the Tank Crusher's 21 Ton Collision, the Wandering Samurai's 9-slice Attack, and the Mega Tank's Tank Shell are all good options.  It's difficult to rank these attacks from best to worst because it will depend on what rank the units are, so you'll just have to experiment a little to see which does the most damage for you.

Other Tips:

Don't forget that retreating is an option!  If the initial part of the battle doesn't seem to be going well, just retreat and start over.  If you can't freeze and kill the ranged arm quickly, or your Radio Tech gets taken out before it can attack, or your Legendary Boars are not hitting for much damage, just start over.  If you like to use Radio Techs, I recommend you get several so you can restart right away instead of waiting for them to heal.

The two AS body attacks have a warm up requirement of 4 and 6 turns so initially only the arms can attack.

If you don't mind spending a few nanopods during the battle don't forget that you can heal 100 hit point per turn, which costs one nano each time you do it.  There will be times when every single one of your units is stunned or waiting for an attack to become available again.  Those might be good times to use your turn to heal a unit.

Everyone takes a slightly different approach to battling the AS.  Please share your effective strategies in reply to this post.

It might be a good idea for those who beat the Ancient Sentinel regularly without using nanopods to post their formations on this thread.

Even if you lack certain of these units, knowing the formation can be helpful. Try replacing the units you do not have with similar units, which you do own.

I have never used nanopods.

This is the formation I currently use, and have used with about a 95% success rate since I acquired these units:

Front Row: Melee Fragment, Legendary Boar, blank, Legendary Boar, Melee Fragment
Second Row: Legendary Boar, blank, Mega Tank, blank, Legendary Boar
Back Row: Mega Tank, Legendary Boar, Radio Tech

*Before I got my Melee Fragments I used Mega Tanks in their place
** Recently I have been experimenting with replacing the two remaining Mega Tank slots with other units such as the Bora HC and the Tank Crusher.

Front row:    |Legendary boar|--------------------------|Legendary boar|---------------------------|Legendary boar|
Second row: |Tank crusher....|Boar 2.....................|Tank crusher.....|Mechanized trooper|Tank crusher....|
Third row:    |---------------------|Wandering samurai|----------------------|Wandering samurai|---------------------|

* '-' means there is no unit there.
*'.' Is just to fill up so it looks nice and easy.
* I indee don't use the radio tech, I think it is too fragile and the boar II makes up for its damage.
Tactic: I start of with the mechanized trooper, doing around 150 damage with its third shot.
Second turn I freeze the left ranged arm from the as with a legendary boar's last attack.
Third turn I kill the left arm with the boar 2 plane.
Fourth turn I have two options: if the arm isn't dead I use the 2 ton collision attack from the tank crusher to jump on the arm and hit the as at the same time. If it is already dead I use the mechanized trooper again.
After that I use this:
Mechanized trooper as best, if mechanized trooper isn't ready to shoot I use lb freeze attack, and return to mechanized trooper. If both the mechanized trooper and the lb are out of hits, I use my boar 2 or the wandering samurai (the boar 2 if he armour is already gone, if the armour isn't gone I use the samurai)
If later some units die and the samurai, the mech trooper, the boar 2 and the legendary boar are all out of moves, I am starting to use the 21 ton collision of the tank crusher.

I lost 1 unit: the wandering samurai

Today I tried using Mech Troopers against the AS.  I was very impressed with them for two reasons.

1.  Their Power Crush attack averaged about 150 HP against the AS body.
2.  Their immunity to being stunned resulted in them being available for the entire battle.

In today's battle I used the following formation and only lost one unit, my Elemental Trooper.

Front Row:  blank, ET, blank, LB, blank
Second Row: MT, LB, Mech, LB, MT
Third Row:  Mech, Boar II, Mech

I froze the Ranged Arm and killed it with two Power Crush attacks.  All other attacks were against the AS body.  I did not bother to stun the Melee Arm.  I did not use the Mega Tanks at all.

I haven't had a chance to use the Mech Trooper against the AS yet, but here is the formation I use:

first: blank, MT(R6), MT, MT, blank
second: LB(R7), LB, blank, LB, LB,
third: BoarII(R7), LB, RT(R5)

I go straight for the body. I beat the AS ~80% of the time. If I find my LB's aren't getting 120+ dmg each in the first few rounds and/or my RT goes down before it can fire, I usually withdraw and start again.

I realize this formation is heavy on a nano unit no longer available (the LB). When I am short LB's, I usually sub in a Wandering Samurai(R5). You could also replace with the new Mech Trooper.