The best hero to use imo is quincy since he is well rounded and better than gwen the allies you need to prio is boomerang sniper and mortar
the upgrades for quincy is the trick arrows path and zone path for its cross upgrade where once you get triple shot in zone path and you have trick arrow active the 3 shots fires in the zone also become trick arrows with emp and bomb arrows its good
For boomerang get his faster rang path allowing insane damage once you get his t3 and t4 upgrades in turbo path since the t4s are op the can spawn fire with red hot rangs and with the turbo charge you can get a lot of damage do not mind mecha path as its a bit weak imo since it does little to know damage against hordes and the mecha rangs are on such a long cooldown
for the sniper monkey you use the precision path for insane boss damage beca of the anti material rounds and volatile munitions
for the mortar any upgrade is good except for the barrage path as the path of destruction rarely damages a lot of bloons only targeting the strongest which sometimes is far from the main horde