This isn't a luck-based achievement...there is a technical for more than 15 objectives during the same season(18 exactly).
TIPS-Play WRC mode in carreer mode...big calendar,big opportunities(all of 13 rallies)
AUSTRALIAN DON'T COUNT...SPAIN RALLY IS THE LAST!!!-Season objective not count.
-When you change objectives and it's not good don't push the B button->Autosave

...go to the
dashboard,quit the game,reload and retry until it's good.
- You need at least 2 weeks to get new objectives after completing them.
-Don't forget the event "maintenance" to do every 2 rallies maximum...for me I used the skill involved
in crew(yellow part)...5 events for week.
-Play with no damage(just visual)...important for small objectives.
-Don't use objectives "don't use soft/hard tyres"...be dependent on the climate is not good
-For small objectives use "Don´t get more than x minutes repair penalty" (call A)
"Don´t get more than a minute penalty in a rally" (call B)
" Do not spend more than $x on repairs in the next rally"(call C)...on damage just visual,it's easy!!
-And the most important for medium objective just only use "Perform at least 2 rest events in 6 weeks" (call D) no need to do objective with 2 rallies,it's faster
PROCESSJanuary Week 1 -change small objective for A,B or C
-change medium objective for D (insist,sometimes it can be long)
-launch rest event(always pop

)
Week 2 -launch rest event(first objective)
Week 3 -like you want...for me rest event
Week 4 -Monte-Carlo(2nd)
February Week 1 -change medium objective for D
-launch rest event
Week 2 -launch rest event(3rd)
Week 3 -change small objective for A,B or C
-Sweden(4th)
Week 4 -Maintenance
March Week 1 -change medium objective for D
-launch rest event
Week 2 -change small objective for A,B or C
-Mexico(5th)
Week 3 -launch rest event(6th)
Week 4 -Corsica no objectives
April Week 1 -change small objective for A,B or C
-Maintenance
Week 2 -change medium objective for D
-launch rest event
Week 3 -launch rest event(7th)
Week 4 -Argentina(8th)...........................and again and again and again
Unlocked on the 11th Rally(Turkey-September Week 3)...if you are wrong,you have
5 weeks left including 2 rallies(Wales and Spain)
Sorry for my english and good luck everyone!!