Yay, so after going close several times (a couple of 5k + damage games) I managed to do 6k damage in the M48 Patton (man I love this tank) to finally clear MT13.3 for the Obj 260 missions, which thankfully also finally cleared the stupid Alliance 1 for the Chimera which needs you to do 25% of your team's damage in a win, so can move on to the 15 mission for that set as well (now at the 15 missions for every group except for Bloc for Chimera). Slowly making progress.
So after many years, the first of the 15.3 missions for the Object 260 has been completed... and it's LT15.3 - probably the last one I would have thought I could complete back when I first started out. 2k damage, 7.6k spotting courtesy of the stealth build on Ghost Town encounter mode. Got it with honours too, which is nice. I have TD15.3 unlocked but realised that 8k damage is just insane and I have never done that in a TD before (that I can remember) so that could take some doing. I'm stuck on the 3x HP blocking mission (with 3k damage) for HT, which is usually my best class and in the past the set I usually finish first, and stuck on a 6k damage medium mission (also very high performance for me in a medium tank, only hit that mark in a handful of meds). Could take some doing, but it just shows I think how powerful the stealth build is given it made LT missions feel very attainable with the right map.