The Avatar has a decent set of tools for engaging infantry models with a Multi HMG, Sepsitor plus, DA CCW. With its BS 15 and ODD the Avatar can out shoot almost anything. It is vulnerable at short range though against models without Cubes since it cannot use it's Sepsitor against them. This can leave the TAG with no option but a HMG in poor range. Though it has a DA CCW and a CC of 18 the Avatar wants to avoid melee whenever it can, since even mediocre martial artists can take it down.
The Avatar's Optical Disruption Device is a fantastic defensive tool granting enemies -6 to hit it with BS attacks. If that were not enough, it has ARM and BTS of 9 which become very hard to overcome in cover. In the rare case of Guided Missiles, it can use its ECM to defend itself, and last but not least, it is immune to Possession .
Sepsitor can be used as an Intuitive Attack from smoke to allow you to keep yourself safely out of sight and also make it hard for the enemy to dodge (they will do so at -6 due to the visibility penalty. The Avatar in Suppression Fire is a really obnoxious thing to have to remove. With G: Gnemonica, you can use the Avatar's ghost (after it dies) to buff any friendly trooper with cube or Autotool. This can really help a Shrouded FO or an Umbra Hacker to dominate the field, but it is also surprisingly effective on a Unidron FO or on the M-Drone that can then Sat Lock on 14s The Avatar has access to a Staldron remote that can let it accomplish objectives, and will not leave it helpless if it is lost to enemy ARO.
The Avatar seems unkillable because it is so hard to hack or take out with small arms, but there are some strategies to pursue. In melee, it can be tied up or even taken out by even a minor close combat expert with litter more than an AP CCW. Even if the trooper can't really hurt it, leaving a model in base to base with the Avatar allows you to set up quality ARO against it or to simply have to spend orders getting out of close combat. Just don't forget about Sepsitor! Be careful on the approach. It is ill advised to try and shoot the avatar down while its ODD is up. This can be burnt off with Fire ammunition, and makes taking the TAG out a little easier. Weapons for removing it include anything with AP, Monofilment, Adhesive, or K1 and the like, which negate some of its massive armor. Visor troopers are also very helpful as long as the ODD is still up.
Deployment with the Avatar is difficult in two ways. Firstly, Strategos 3 is a public skill announced at the start of deployment so your opponent will know the Avatar is coming. Secondly, the Avatar is pretty big. General places that it will do well are those where it can start from - and return to - total cover.
Active Turn Role
The Avatar is THE active turn piece. With it's HMG and Sepsitor it wants to do as much damage as it can before retreating to safety. While the idea of Sepsitorizing the entire enemy force is fun, its better used to take midfield skirmishers then use them to accomplish objectives. With its Staldron and weapons though, it is quite possible to spend all of your orders on the Avatar and be in a good position at the end of the turn.
With its stat line the Avatar often wants to shoot against enemy models, or sepsitor if they are getting close, but keep in mind that sometimes it can be worth dodging. You will take a -6 penalty, but that still leaves you will 11 PH and success can get you to safety. Especially consider this against models with lots of defensive modifiers like ODD or Camouflage.