All Primary Monkeys in radius get tier 1 upgrades for free, increased range and reduced ability cooldowns.
~ BTD6 description
Primary Mentoring is the fourth upgrade of Path 1 for the Monkey Village in Bloons TD 6. It allows all Primary Monkeys within its radius to be granted free Tier 1 upgrades, alongside the benefits of Primary Training. Starting from Version 17.0, it also grants nearby Primary Monkeys with +5 range and reduces their ability cooldowns by 10%.
Primary Mentoring further improves exclusive buffs for nearby Primary Monkeys by extending the Primary Training effect to also include free Tier 1 upgrades, reduced ability cooldowns, and even more range. In addition to strategical advantages provided by Primary Training, the allowance of free Tier 1 upgrades makes it less costly to spam in more Primary Monkeys within range, in addition to allowing Primary Monkey abilities to be activated more often. Since the Primary Mentoring range buff is additive, rather than multiplicative, its extra range benefit is greatest on low-range towers, namely lower-level Ice Monkeys and Tack Shooters.
The value of the Primary Mentoring upgrade benefits makes this upgrade see its highest use in combination with Striker Jones + MOAB Mauler spam strategies. The free Tier 1 upgrades portion of the upgrade allows more MOAB Maulers to be added within range, saving a significant amount of money in the long-run to compensate the expensive supports provided by the Village. On the other hand, the MOAB Maulers benefit well from the extra range and all other preceding Village supports, while the reduced ability cooldowns increase the viability of MOAB Assassin abilities versus spaced blimps. Other popular supporting Primary Monkeys also can benefit much from this upgrade's various supports, specifically MOAB Glue, MOAB Press, Bloon Impact, and 0-1-2 Re-Freeze Ice Monkeys.
For the free Tier-1 upgrades, Primary Mentoring is best used with Bomb Shooters, as their Tier-1 upgrades collectively are the most expensive of their kind.
The two most expensive Tier-1 Bomb Shooter upgrades are Bigger Bombs ($350 on Medium) and Faster Reload ($250 on Medium), saving up to $600 on Medium.
As such, certain Striker Jones + Bomb Shooter strategies, particularly with either masses of MOAB Maulers or Recursive Clusters, will benefit the most from the discount portion of the Primary Mentoring upgrade. For 2-0-4 Recursive Clusters, this discount is slightly less than that for 1-3-0 MOAB Mauler spam, but each 2-0-4 Recursive Cluster can save to $550 on Medium ($350 + $200, Medium prices) per Bomb Shooter if upgraded within range of the Primary Mentoring.
Because of the tiny ranges of most Ice Monkeys and Tack Shooters, the +5 range buff is much more significant of a range increase for those tower types.
This especially empowers the Super Brittle + Tack Zone synergy, because these two towers hand-in-hand support each others' weaknesses; Super Brittle is a poor damaging tower on its own, but its massive damage debuffs against affected bloons plus its moderately adequate stalling power against Super Ceramics allow the Tack Zone to reap up large amounts of damage, especially against Super Ceramics, the bloon types that Tack Zone may typically struggle heavily against.
Generally speaking, the effective crosspathing for the Primary Mentoring depends on the situation. In most cases, however, a 4-2-0 Primary Mentoring is more desired to grant camo detection to nearby Primary Monkeys, although 4-0-2 Primary Mentoring is not a bad option for further saving a good chunk of money for more expensive lower-tier Primary Monkeys.
Previously, Primary Mentoring did nothing outside of making Primary Monkey Tier 1 upgrades cheaper, which had little to no effect outside of supporting lower-tier Bomb Shooters. Primary Mentoring was buffed in Version 17.0 to add more buffs to other aspects of Primary Monkeys, specifically additive range and reduced ability cooldowns.
Primary Mentoring now reduces ability cooldowns of primary monkeys in radius by 10%.
Primary Mentoring now adds +5 range to primary towers in range.
Primary Mentoring as seen in the upgrades menu
Primary Mentoring upgrade icon as seen in upgrade menu