Thanks for the report. I have to ask a rhetorical question; if you were intending to use the Warlock to summon the Avatar, which turn would you have done this, and what effect would this have had . . . . ?
As far as I can see, the Warlock spent the game cowering in the far left corner, and then moved up the left flank assisting in the destruction of the Tank Company (which becomes a Heavy Tactical mid assault

), before shooting the central artillery company. So, at no point is it in a sensible location to call in the Avatar as there is nothing near it to destroy at the point of summoning . . . .
If you use a Warlock, I recommend placing it opposite the enemy Blitz or the Eldar T&H objectives (both if possible), in such a way that it can hop from one piece of cover to the next (to reduce the chances of being hit). Then you place at least two warhosts and one or two support formations such that they can all join up on the middle of the table for the start of turn #2, within assault range of something juicy. At this point you summon the Avatar and let rip as they say, preferably using the triple retain. Ideally the Avatar will lead a warhost into assaulting one formation (supported by the Warlock), both consolidating to support the Warlock's assault on the next target, and then have the Warlock consolidate to support an assault on a third target. . . . .
Note, even if the enemy target the Warlock (as is likely), if it starts in the right place, it can still marshall forwards while removing BMs, putting it within 20cm of the middle of the table and possibly in range to summon the Avatar near other formations. Also, this tactic provides a quite decent AA umbrella for the associated formations, quite apart from any other ground AA
On a separate note, I would also recommend placing the guardians behind their Wave Serpents when initiating an assault so that they take the initial hits generated by the enemy. In the 2nd turn mounted Guardian assault I accept that you may not have had the distance to do this in the first round, but at the start of the 2nd round the Wave Serpents could have counter-charged forward to the front of the formation, which would probably have won the assault there and then.