This is a small tip i recently started to use consistently and it helped me a lot for the node 11/12 battles. I'm sure many of you know this already but for some others this might be helpful and to the best of my knowledge it wasn't explicitly mentioned here on the forum (at least not explicitly in GW guides).
A generally applied strategy for the last battle(s) in GW is to use suicide or B/C/D/.. squads to set specials of the opposing team on cool down and to weaken them. So the last battle(s) will mostly involve several teams of your own. However, the problem is that any new character brought into a GW battle will not profit from the TM carryover from previous battles rendering them, together with the fact that those mostly will not be A-team chars and therefore most probably slower, in a disadvantageous position.
This can be countered by using your suicide and C-/B-/ (etc) Teams on node 1,2,3 ... (starting with the weakest team) and only starting to use your main squad when nodes become stronger(around node 3/4; that's important because otherwise the protection of your secondaries will be damaged to much). This way, the TM of all your chars relevant for the later hard nodes will be pre-charged. If re-introduced in a later battles they will most probably get turn before any opposing char which helps significantly.
Of course this only applies if you have secondary teams who can beat those easy nodes without loosing a char and (at least) with full health (better almost full protection).
This rule can also be applied if you follow a different strategy, for instance saving your A-team for last node (then you would use this team at node 1).
So the key is not only having a large rooster at the last node but also to ensure that each character has pre-charged TM by that time, i.e. used at least once during prior fights. For me, i was somehow aware of this long ago but didn't care that much because i was too lazy to switch teams so early in GW. But preparation is what makes the later nodes easier and that safes much more time. So consistency is important too!